It may help his case, but it is not of monetary or special privilege to him personally. It is a time honored way of obtaining testimony against bigger fish.
Promises of money or special privilege to a prosecutor is never seen as pursuant of a conviction.
While you could say a feather in his cap is a personal gain, it’s not really the same thing.
He also gets to keep himself at the public trough for some more time. Mueller is not doing this for free even though he is old enough to be getting his gov’t pension. (Which is a couple of good questions)