How do you know when to use "lequel" in French?

I'm aware that it can be used in the sentence:
"...les raisons pour lesquelles" but this is the plural form, and the only time I've felt it's appropriate to use, but I'm not sure why it's used, when surely you could just write "les raisons pourquoi"
Can anyone explain when to use laquelle/lequel/lesquelles and maybe try to give a memorable translation of what it means, and examples of different places where you'd use it?

LEQUEL
In relative clauses, lequel is used for specific inanimate antecedents

---------> La raison pour laquelle il est venu. (laquelle referring to the clearly defined concept of 'la raison')

(For specific animate antecedents we use 'qui' - La femme avec qui il est allé au cinéma)

QUOI
quoi is used for neutral antecedents (ce, rien, quelque chose, grand-chose, autre chose, peu de chose) or when referring to a whole clause:

"Elle a déposé une plainte contre son agresseur, ce à quoi ce dernier ne s'attendait pas du tout."

"Il a été accusé de malversations, contre quoi il a protesté avec violence." (here 'which' refers to being accused of embezzlement)

In literary writing quoi sometimes is used where lequel is normally expected, but you don't really have to bother with that.