1. It's something new to fiddle and experiment with, and has a solid system to figure out.
2. Backpacks. It's like having a Heward's Handy Haversack once they're all done.
3. Resources. It's not the fastest, but it can supply a (mostly) steady low-output (or, I should say, low-density) stream of resources that doesn't tear up the landscape, and can circumvent the need for mob spawners. Mob spawners still have a higher output and yield experience, but you can get mob drops from bees.
4. Resources again. You can (infrequently) get a few things from bees that are otherwise difficult to obtain.
5. Mob spawners. Apparently. No where near Soul-Shards level, but we accidentally found species of bees that spawn mobs.
6. Giant Tanks of Liquid DNA look cool, and Liquid DNA itself is the topic of (nearly) endless jokes.
7. I'm not sure, but I think they're required for this tree-breeding I've passingly heard about and want to get into later. A big walnut orchard would be cool, especially if you can use walnuts for fuel. A power plant I worked for a while ago burned massive piles of walnut shells because they have great caloric density.