Bumble Bee, (Bombus sp)

I don’t understand the behavior of this Bumble Bee. It was rubbing it’s entire body into the center of the Prairie Rose. It would slowly lean and rotate as it’s body. I assume it was eating, but what? You can see the grains of pollen sticking to it’s legs, but it can’t get nutrition from that. Or does it carry the pollen to it’s hive where it is used?