> Why does everyone want to tax the billionaires?

Because they take a lot of value out of a shared society and concentrate too much power on their hands.

> What are you going to get out of it?

Reduce the power discrepancies, mostly, and money too.

> Do you think taxing the billionaires is actually going to help the poor people?

Yep.

You are asking about government efficiency, but one of the reasons for the lack of efficiency is that billionaires do not want it.