Not an obligation; I consider them about the same as a hi-five.
But you know, hi-fives are nice, and you've been following my work for a really long time.... If a name shows up a lot commenting on my stuff or
ing a lot of it, I usually go check out their gallery in return, and leave a Llama for the heck of it.
Also, to me, helping people upgrade their Llama badges kinda feels like helping them evolve a pokemon... And that's about the most practical use I can think of for them...