We've been asked this same question a few times, and we'd say they seal the ends of your carrier so water, dirt, and Grime didn't work into behind the bearings and such and start to corrode things up..
We also had a TRX450 in here that had been through quite deep water without good seals, and the sealed bearings didn't really hold up like they should've, and the water snuck its way into the bearing, therefore forcing a rebuild..
The seals on the bearings pretty much just hold the grease in. They aren't good enough to hold elements out.
Pack the carrier up with a bunch of grease, so if water tries to get in, it has to push the grease out first! Oh, and as always, lube the seal surface for the best seal and to avoid damaging the sealing surface. YOu can also RTV in the outside of the seal...seals it better than a press fit!