I’ve just started and a bit confused with why we declare some things as separate radicals. E.g. why is Kanji 八 for eight is “fins with an extra line”, but 入 is specifically an “enter” radical, and not 人 “person with an extra line”. Or why is 二 it’s own radical and not “two grounds”, but 三 is “
二 and 一” and neither “three grounds”, nor a separate radical
Sometimes, what WaniKani considers a “radical” is based on what makes a compelling mnemonic. Later on, you’ll see previously-learned kanji introduced as radicals in their own right to this end.
That said, 入 and 人 are actually extremely different. It isn’t clear in the default screen font; it’s more obvious when you see them hand written. Here is enter:
And here is person:
They’re as different as b and d in English.
The 214 traditional radicals give you information about etymology and stroke order for writing, as well as providing a tool to (sometimes) figure out the meaning of the kanji.
So, on the whole, there is a reason, most of the time, but the reasons aren’t going to be terribly obvious until you know more.
Ok, hit level 2 today and confusion continues 
Just learned back to back the “talent” 才 Kanji and the “street” radical 寸 (without the drop part). However, WaniKani says 才 is composed of 一、亅 and ノ, not 寸 and ノ….
The second (vertical) stroke in 才 crosses the first (horizontal) stroke, whereas the second stroke in 丁 (street) doesn’t. Whether or not strokes cross each other is often a critical difference between kanji.
For what it’s worth, 丁 is not one of the traditional radicals; I think it’s one that’s made a radical in Wanikani for the sake of mnemonics, mostly because Mrs. ちょう is extremely evocative.
Not everything Wanikani calls a radical is actually a traditional radical. Many are just kanji made up of simpler radicals that show up as components in other kanji. I think Wanikani uses like 500+ “radicals” but there are really only 214 traditionally recognized radicals.
丁 is technically not a traditional radical itself but is composed of traditional radicals 一 and 亅. It is a common element found in several kanji so it is somewhat useful to learn as its own structure because it can inform about stroke order and stroke placement in those kanji.
才 is composed of 一、亅 and ノ but has different stroke placement than 丁. Stroke order and placement are extremely important in kanji.
寸 is a completely different thing and is also it’s own traditional radical.
