Phoenix said "There is a limit on the amount of characters...."
That's for sure... I've run into problems because I like to make my signature line smaller, in italics and a different color. Several times, I've been informed by the system that I had run out of room. Then it's a scramble to make it all work.
The reasons for my signature lines are as varied as the lines themselves. I like change and poetry that speaks to me. For a while I changed my signature line every day. I copied, for example, almost the entire Millay poem, "Renascence," over the course of a month, one couplet at a time. I don't think anybody else noticed but I didn't care. I do things to please myself.
My current line is from the Bible... odd for a Pagan, I know, but this season is all about light. I love my signature lines enough that I save most of them on my profile page. I just disentangle them from their formatting code and drag 'em up there.
I've used a few again. Who knows, god forbid, maybe I'll run out of good lines.
I also love the image of my signature line being changed, not just on all the current posts, but on every post down the line from the beginning of the forum. Technology is 'mazing.