Obviously, they can do many things.. and so, while I played I had individual sigs for a number of different reasons. Over time though, they do communicate some essential things : something about me as a player, and something about me as a character.
Below you'll find a sampler of some of my sigs.
That's one of my earliest sigs, from the time my character was still affiliated with the Blood Raiders.
I think the key here is
attitude... and humor.
Starsi was a popular thing at some point, so here I'm showing my support. And I guess ever since that, it's been "GoGo Yubari... sponsored by Starsi!"
Representing the Intaki.
Here I wanted to try a different visual style. More chaotic, more messy and with the tagline I've since associated with GoGo.
Of course, sigs also reflect your in-game allegiances and you take one of those "group sigs" as your own.
... and sometimes they do that while preserving your own particular flavor. That's actually probably still my favorite. The mood, the colors.. and an RL friend of mine in the pic.