There were some valid reasons to have them, if you go far enough back in history, but those reasons haven't applied for a long time. Meanwhile, signatures gradually evolved into a plague. They often took up more space than the posts and they were often brightly colored or animated. Sigs made reading the actual text harder while serving as a distraction.
As you might guess, I am glad to see them gone. What used to go into sig blocks can now go into your profile. Anyone who's interested can just click. It's a much better setup all around.