I think the purpose if impeachment is to protect government of, by, and for the people from infection by identifying and accusing bad people who either are in positions of high public trust, or were in such positions, and while there, showed that they were willing to abuse that trust. Some such bad people are current office holders. Some were in office but are no longer in government. In the former case, a Senate trial leading to a conviction both takes the convicted person out of office and prevents them from ever going back in. In the latter case, the latter consequence only is visited on the former officeholder in question, which is no minor consequence, and probably easily justifiable in many different situations, just not with Trump.
The purpose of impeachment is to prevent law enforcement from overthrowing the federal government by arresting executive, legislative or judicial personnel.
Unless one of the above is removed by impeachment first it is treason to arrest them.