Joining Battlegrounds gets you rank points when killing enemy players. It's not just about the loot from towers and boxes, if you are a pvp person you wouldn't think it's boring. Although sometimes if many players leave after looting then it may get boring because a certain unbalanced bg may become even more unbalanced, leading players to give up winning.
Hello Aegis! How's life without SB? Hahaha hope you will return home soon so that I can see you in game
Anyways, BGs allows you to gain points without the fear of losing them, as even if you die you won't lose rank points like in Caergate. That said, as a consequence, the amount of rank points gained in BGs are significantly lesser as well.
You are right though, other than getting loot and exp, there really isn't much for someone who prefers to PVE or someone who is looking for an objective. Maybe this is something that can be looked into by aeria games, and they can pass on the idea to the developers.
This is still only the first BGs that are available to us, I am sure down the road we will be seeing more interesting ones as well. The only question is when new content will actually come