[lyall]

The radicals and mnemonics are useful for bridging the gap between learning a word and truly acquiring it. Think of it like learning how to read in English: when starting out you have to sound out every letter, recall the weird rules about how sometimes "ough" can make an "oo" sound (or was it an "off" sound?), use your finger so you don't loose your place etc. But eventually with enough repetition your brain sees the jumble of letters that is the word "throughout" and immediately knows what it says.

Everything that WaniKani does is just to help you get from seeing "日本語" and it meaning absolutely nothing to you, to seeing it and immediately knowing what it means. The radicals and mnemonics are all secondary, and don't need to be remembered.