and it's possible for him to be tried in The Senate even after he leaves office
The value of doing so is that he can be barred from holding public office and stripped of his post presidency perks such as his pension, travel budget, secret service protection, etc...
It's not like he's going to really need those things anyway given that he's likely headed to prison or exile in Russia
It would require two thirds of the members of The Senate to impeach him....but only two thirds of those members that are present for the vote so Republicans who don't want to get their hands dirty defending the country from a domestic threat can just skip the vote