This is only really true if you're already educated. Presuming that reading and writing is something that all can do would certainly lead you to believe the dictionary had limited usefulness.
Wiktionary is part of the same project as Wikipedia. It provides dictionary-style definitions for basically all slang terms whereas Urban Dictionary is actually terrible if you want to find the meaning of words because half the definitions are jokes while the other half define slang in terms of other slang.
Yep, 'watermelon sugar' is still a disputed hellhole and I feel like the UD editors make it worse by approving everything there to drive up engagement.
There's so much clear editorialization of terms that legitimate definitions are flooded out for what's trending at the time, so nothing is up to date, the editors scrape by for engagement, and its not useful anymore.
As an adult sure. But when I was a kid I loved my dictionary. I learned so many words that I might not have. And I enjoyed books much earlier than I would have.