One arguement for Impeachment that has been put forth is as follows. First, you are right about the Senate not voting in favor of the Articles of Impeachment. But the action of Impeachment legitimizes the charges and the investigations.

By bringing forth the evidence that will undoubtedly come out in the proceedings, would serve to impress upon people how corrupt he is, as well as how unfit he is to hold the office. The hypothesis being that the exposure will embarrass and dissuade his marginal followers to abandon him. His core is too incorrigible to care about his criminal activity.

I don't know if I buy this reasoning, but its worth mulling over for a little while.