何必【hébì】

何必 goes before the verb and makes a rhetorical question: "why would you need to?". Nobody expects a reason in reply; the speaker is saying it's unnecessary, and often trying to talk someone out of something. A 呢 at the end softens it.

Giving the reason first

何必 usually follows a clause that says why the action is pointless – it's close by, it's late, it's only a small thing.

何必呢 can also stand alone, as a sigh at someone who is overdoing it: "what's the point?".

何必 or 不必

不必 says the same thing as a plain statement. 何必 puts it as a question, so it sounds more emotional and closer to a gentle reproach.

Asking for a real reason

何必 never asks for a real reason, and the listener isn't expected to give one. To find out why someone has to do something, ask with 为什么.