What can we help you WITH ?

hi, everyone

Could you help me explain why

How may I help you / May I help you ? (without with)
but : What can we help you WITH ?

Why we need to have a prep. here and cannot drop it off ?

The non-inverted, non-question form of the latter sentence is “We can help you with X”, where X is some noun. When this is turned into a question, X is replaced by the pronoun “What”, and the sentence is normally also inverted so that “What” comes at the start. That makes “What can we help you with?”.

The sentences “How may I help you?” and “May I help you?” do not explicitly include the “with X” part, so there is no “with”.

Thank you Dozy, your explanation is very clear.

Would “How to help you?” work? Thanks