Is "she stepped off of the street curb." grammatically correct?  If it is, why is 'of' needed?

'Of 'isn't needed. Off is all that's needed. 'Street' isn't needed either. Streets are the only things I can think of that normally have curbs so if you say curb it can be taken to mean a street curb. If she's stepping off a curb that is not a street curb, then you'd want to say what kind of curb it is.

Or you could say "She stepped off from the curb".

Technically 'of' is not needed as 'off' and 'of' are both prepositions and two prepositions side by side are not needed.

In this phrase, "of" is not required. It would be fine to write "she stepped off the curb." Even "street" is not really required and could be eliminated. If brevity is the goal, deleting "of" and "street" would be an improvement.

She stepped off the curb.  It's fine without 'of' or 'street', but it seems incomplete.