They don’t.
They KNOW that there are no grounds for impeachment, however, they have to satisfy their rabid HATE TRUMP base (see Hollywood for instance).
So, even though they KNOW that it will go nowhere when it reaches the Senate, they want to send a message to their rabid base to in effect say: “See, we tried.”
They also want to forever tag Trump with the prefix, the impeached President, which the mainstream press will henceforth, gladly usem