The signature line helps organize a person's post. Without it, it would be confusing to tell where the post stops, as some people's signatures are just plain text or something.
As for BBCode, it's for more than, er, Wiki stuff....BBCode is for controlling the actual look of the text in your post. If you make a post, on the top of the textbox is a whole list of little boxes with various BBCodes--B, i, u, Quote, etc. The actual code shown upon clicking those buttons is the BBCode.