A couple of reasons.
The greater the number of roles a player has experience with, the greater his usefulness since there's a greater number of situational responses he can be plugged into - either as a direct producer, or as a distraction/misdirect. So, by giving Patterson more experience actually running out of the backfield, it enhances his individual value and establishes him - in the eyes of opposing defenses - as a legitimate threat to run.
He had 10 carries in one game, which is more than he's typically gotten in one season in the past (yes, he's been used as an RB a handful of times during his career prior to joining the Pats). It doesn't matter that he failed on a few of those. What matters is that he was at least marginally successful on a few, too. But, what matters more is that McD kept running him. His first appearance in the backfield against the Bills turned out not to be a one-off "trick play", so the defense had to begin playing the run when he was in there.
So, all of that is now on the tape that the defenses of future opponents will be studying. It's another possibility that they'll need to account for. It's possible - maybe even likely - that Patterson only takes a couple-three handoffs from Brady between now and the end of the regular season. And then, in the post-season, McD will be able to line him up in the backfield and run play-action off that. To a great degree, it's a "setup" for a future opponent - something the Pats do regularly.
Anyway, I suspect that McD knew from studying tape that the ground game wouldn't be particularly successful against the Bills D, whether it was Barner or White getting the carries (and it wasn't), so giving handoffs to Patterson wouldn't be significantly worse (and it wasn't), plus, doing so would help establish a set of misdirection plays for future games.