I'm sure I wrote the same response almost to another post a few months back as someone else asked the same question about using a CSS Framework or Bootstrap, but even now a few months after I would still agree on the same opinions.
Using pre-built frameworks can expand your knowledge, but they are more or less for templating, popup fast websites to get them into production ready. Some of the popular ones are BootStrap, Material CSS, Skeleton, and Bulma.
Only catch is to using pre-built frameworks is that they can be alot harder to customise. I prefer using Skeleton and I even downloaded the raw css files the other day and after a few lines of code it put me off wanting to do a customisation, due to the css rules are already set and its a pure headache for me.
I've just been working on a new theme for my own CMS and already its taken me at least a week, to get the features and the customisation I want but at least I know in the future I can just adapt what i need to and hopefully should be a lot faster the next time round.
On another note I would mention is that if you're an old school div person like me and still getting your head around to these new grid and flexbox features. I've been using a tool I found online called Grid Layout, Building simple layouts is fine but more complicated layouts I have noticed does not all work with CSS Frameworks.
Thankfully I understand the grid layout now, so should be able to do it manually in the future :)
Pros for using Frameworks
Cons
Hope this gives you a bit more knowledge, and never think of anything as embarrassing, it can be a learning curve :)