Kevin C.
Well, the whole thing isn't placebo, but what parts are not do little more than fill in blanks WP should have done out of box.
For example, legitimate things it lets you do is edit the description and keywords META... but since description has zero impact on weighting and simply exists as text to show beneath your TITLE on your SERP listing? Hard to call that SEO when it should default to your first content bearing element inside MAIN (if present) or after the first H2/HR. Keywords on the other hand should only be seven or eight single words that exist inside BODY, and it does little to enforce that resulting in keywords being more likely ignored that useful.
Side note, despite the wild claims a well crafted keywords META with content relevance, under 72 characters in length, with no repetition of words inside itself is not ignored by search! Even Google seems to have forgotten how its own mechanism of action works. Its only when overstuffed with irrelevant content that it is guaranteed to be ignored. It's called keywords. not keyphrases, not keysentences, but keyWORDS!
But from there? It tends to be utter and complete nonsense.
The outright scam that is "keyword optimization" for example is where the black-hat SEO con-men thrive. Cherry picking words people are searching for and shoe-horning it into your content may get you to rank for a 'magic seach phrase' increasing traffic, but if that traffic is irrelevant to your topic becuase those terms were just forced into the copy, it's wasting your bandwidth on visitors who are unlikely to be turned into conversions!
I knew a guy about a decade ago who was always bragging about the great search results his keyword optimization had for what I call "perfect magic matches" -- that had not a single thing to do with his topic. The pinnacle of this was his bragging about how what his l33t skillz had put a site selling a Photoshop plugin at the top of a search for a certain phrase... Which it did.
Too bad every other website that came up was about powder coating automotive engines. He had failed to recognize or understand the marketing and advertising concept of "Non competing competitors". People searching for the phrase he was bragging about weren't looking for his topic!!!
It's hoodoo-voodoo that yes, gives you a top ranking for particular phrases, but that does not mean those phrases have anything to do with your site or generate traffic of value. SEO scammers use this one all the time, to show off how great they are at getting you top ranked on Google, and clients/site owners who don't know any better are conned by this LIE every day. Anyone can pick a phrase nobody is actually using for a topic, and get top ranked for it. That's not SEO, that's placebo.
The setting of canonical URI's is another example where either wordpress is utterly and completely incompetently coded since a CMS out of box should prevent that from happening in the first place, or they're just putting it in to brag about "look what we did". Either way 90%+ of the time setting the canonical META is a waste of bandwidth.
Readability checks are possibly one of the worst things in the industry. This sounds odd, but far, far too often I've seen people posting technical topics who take the Flesch score as the gospel, and as a result dumb down their content to the point it can no longer convey the topic at hand. By insisting on reaching a "wider range of less literate people" they fall into the false simplicity trap. Well written relevant content more often than not scores low on that test for a reason.
It also wastes time with that sitemap.xml nonsense, something that if you have a CMS and your entire site is internally cross-linked is 100% sugar pills. The ultimate in placebo nonsense that serves no purpose if the site can be spidered successfully. Submitting sitemaps to Google, creating them expecting them to do anything in terms of SEO is ridiculous, redundant, and serves no legitimate purpose other than making people waste time to feel good about their alleged skills.
Much the same can be said about screwing around with "robots.txt" or the robots META. Generally I find setting off-site links to nofollow to be a bit... hinky. ... and if you don't want search indexing pages 1) why did you put it online, and 2) why did you put it online. Of course that it allows/creates "INDEX" and "FOLLOW" values that do not even exist as valid robots values -- and that so many people blindly copy said invalid practice? Colour me unimpressed. That in some cases the nonexistent INDEX and FOLLOW were at one point accidentally treated as NOINDEX and NOFOLLOW by Baidu and Yandex? Classic. No, Google is not the biggest most popular search engine in the world.
Of course all of that is silly little window dressing on the SEO front compared to semantic markup, logical document structure, and unique content of value. In fact since the use of researched keywords, dumbed down language for "readability", and so forth can result in your content being made as uniform and bland as everyone else's, it in fact often negatively impacts relevant search rankings.
Which is why my buddy Dan often referred to such things as "dumping a can of shellac on a pile". It polishes everything up nice and shiny, but that doesn't mean you actually fixed any underlying issues or actually impacted your SEO in a meaningful fashion.
But because it's -- like so many of these media darlings -- touted as a SEO solution, nobody wants to hear that their sloppy off the shelf template with the gibberish markup, mated to bland cookie-cutter dumbed-down copy is what's holding them down on the SEO front, instead saying "But I installed Yoast, it's taking care of that".
No, no it isn't. Not by a long shot.
On page SEO is simple. Ridiculously simple... so simple that it's disgustingly dishonest that there are people who "specialize" in just that, resulting in a cottage industry having grown around dimestore hoodoo snake oil peddling. All you need is unique content of value, marked up semantically. The former being the entire reason to even have a website, the latter being part of the many good practices you should have from the start.
Both of which play to the most important search advice we ever got, from Matt Cutts himself:
Write for the user, not the search engine.
... and sure a good description meta can make your SERP listing look pretty, and doing keywords properly can help a tiny, tiny bit; but if the CMS isn't offering that out of box from day one instead requiring you to use a plugin to add it? Well, then said CMS is garbage that failed to provide the most basic of HTML construction functionality.
Almost as if the people who created it didn't know enough about HTML to make a content management system. A conclusion supported in equal measure by their server-side code.
As I've said several times, "for people who know nothing about websites, by people who know nothing about websites" is a pretty lousy plan.