Well, most of the time, a sig that you make yourself ends up looking better because it's what you want. If we create one for you, then we don't know what your vision of a "perfect" signature is, thus upping the chances that you won't like the style, colour, etc. This way, you can be as specific as possible, so you get something that is closer to being your creation rather than the shop owner's. For example, just take the siggy I recently made for Peefy. He requested, and I made, but it wasn't exactly what he was expecting. There's why you need to be specific.

Of course, if you want something completely random, then that's fine as well.