Exposure to new beers, and availability of beers I can't get elsewhere.
Does your "growler fill" permitting allow people to drink on-premise? Sorry if it's an obvious answer, the growler laws are insane out here in Cali. They're only just recently considering the idea of letting one brewery put their beer into another brewery's growler, so the very idea of a store filling growlers with beer from multiple breweries is kinda blowing my mind a little.
...anyway, if you can, host tastings/tap-takeovers! It's fun to taste the "B-sides" in a brewery's line-up, or taste examples of a style that aren't the ones you'll see for that style in every bar. Even when the actual tasting doesn't blow me away, I have a hard time leaving my fave bottle shop without three figures of something every time I walk in the door.
Oh, and, ditto what everybody else is saying on the BMC tap. Stock the cans, but the folks buying BMC aren't the kind of beer geeks who will care enough about getting it fresher than the cans, especially not if they have to pay for the growler, which they're certainly not already gonna have when they walk in the door.