Language skill and literacy are completely separate skills. People can and do live their lives without ever learning to read. That’s far less common today than historically, but it demonstrates how divorced those two skillsets are. WaniKani is not designed to teach you Japanese, it’s designed to make you literate in Japanese so that actually learning it will be far more accessible.
Think of Kanji like an alphabet of ~2500 core characters. WaniKani teaches you most of those characters, and you’re given a few example words to reinforce it. When I was taught literacy in kindergarten they started with “A is for apple, B is for big booty bitches, C is for crayon…” etc. The WaniKani curriculum is just like that… with 2000 logographs to spend years learning. 
It’s strange to me that so many people seem to have a desire to use words they’ve only seen in one arbitrary study context anyway. That’s how you end up misusing words constantly. Memorization apps and textbooks omit the vast majority of colloquial expressions which are kind of the backbone of conversation. The way I see it is that WaniKani supercharges the ease of all other study, other study enables input immersion, and vast quantities of immersion are prerequisite to actually speaking without sounding like a textbook or a Mad Libs book.