They're doing their constitutional duty. They can't use the assumption that republicans won't do their duty as a reason to not do theirs, at least, not when the crime is this extreme. He incited an insurrection against the capitol that got people killed. You can't just not react.
As a practical matter, at minimum, you force republicans to go on record in defense of this heinous crime. Though unlikely, if he can be convicted in the senate, it'll strip trump of some of his presidential privileges like SS protection and also they can prevent him from ever holding office again.
