Most of the usages of に I've seen have it directly followed by a verb directly attached to it. (The location of existance **にある/にいる**, The action objective **e.g 私は電車に乗りました**, etc)

I know there are some that don't directly have に follow a verb, but its still heavily reliant on a verb **(私は六時に家を出す)**

However, に can also be used when there are no verbs in the sentence 
>私に必要です
 Or
>私には必要です

* I interpret (at least the first sentence) as [smth] is essential to [me] or 私*

I (somewhat) get the fact that に and には mean kinda different things and are to be differentiated, so i will just refer to the first sentence. 

What usage does the **に** fall into here?
Is there a certain way it gets applied in any sentence with only a noun/ adjective (as in it gets used the same way/ means the same thing)

*Other maybe-related examples i found*
>タバコは体に悪い。 
The way i see it (i guess) is that the に marks that tabacco is 悪い towards/ for the body/体. But why can't we use something like が here?

> 私に何か用ですか。
I guess this sentence works in a similar way to the 必要 one, labelling  私 as "What will i use/ be in use for me"

Sorry if this seems like an obvious question, but im genuinely stumped.

(Edited to explain how i see it)