They are just mini vanity pets. There to look cute. Kind of like mounts--there's only 3 different speeds, but no difference between the ones on the same speed besides look.
They don't really annoy people though. They are hidden in Warzones and aren't targetable (through tab targeting) in Raids/Operations so...
And like Ignicity mentioned, if you like to RP or write up back stories and stuff about your characters, it's just a nice touch.
There were 18 pets in game before 1.3. 1.3 added at least 2, though most likely 6 (Some databases found a total of 3 of each species and considering all of 1.2 added new pets in sets of 3, it's likely all 6 are ingame) for a total of 20-24 Vanity Pets.
So there are now 20 useless pets in the game? excellent!! I better re-sub!