Well you pretty much answered it yourself. You can use your signatures to get traffic to different websites, but only if the administrator(s) allow you to have links in your signature. Every forum that I've been a part of has always had that question (If it's allowed or not) in the rules page).

You can also use a signature to express yourself with an image, whether it be your favourite game, band, or random whatever. It can also be a quote as well. But once again, they'll have to be within the guidelines of size and appropriateness as well.