The most common thing I use reduce() for is producing Strings as output, by looping over and processing a list of some kind of data. I might be doing something like:
Array of data into a String of HTML sourceNodeList of elements (like what gets returned from document.querySelectorAll()) in a webpage into a String of CSS sourceCSSStyleSheet into a String of CSS sourceArray of data into other formats (CSV, MD, TXT, JSON, JS, etc) and sometimes also outtputting that as data URIsI think the only time I've used it on numbers has been as a demo summing an Array of numbers to show what it can do, not for anything I've actually needed yet :D