This is really very simple...
If they commence impeachment procedures against this President, he will be barred from issuing Pardons to anybody who could be a potential witness, which would be just about anybody in his administration.
If we don't do this, he will undoubtably issue 10,000 Presidential Pardons before leaving office.
I don't honestly expect this Congress to convict, but just BEGINNING the process will score a major victory for future accountability of war criminals and other Rethuglican evildoers.