Yes, they glue stuff together. And they fill cracks and places that have no use.
But propolis is also antibacterial and probably has more health benefits than we realize. Maybe it's to deter mold growth. Maybe to keep microscopic bad things at bay. Maybe they chew on bits of it when feeling bad. ;) Could you imagine if bees only stuck it around in odd places so it would not interfer with wax comb, but yet had it readily available to eat, feed, and help with overall hive health.
Anyways, there is some interesting research being done on propolis. Seems that breeding bees for little propolis production for the past 50-100 years may be a bad thing for bees. It is there for a reason. And there could be many reasons.
If you take a unfinished piece of wood and make hive bodies with it, the bees will propolize the insides smooth with propolis. They do not like the rough cut lumber. And while we offer bees smooth woodenware most of the time, imagine the interior cavity of that rotten oak tree. I think they remove all they can of the spongy dead wood, and then propolize over the rest. I have seen the walls of feral colonies that have a very slick and shiny coating from propolis. We think that this raw wood in tree cavities would be good absorption for moisture, but bees may be doing the opposite in attempts to benefit from other unknown forces.
I do not look at propolis as a negative. I see it as a sign of hive health, and hope my bees are benefitting from their collection of the stuff.