Signatures are rather like a person's trademark. In fact in some cases signatures such as WK Kellogg, Jim Beam and Walt Disney they are the company trademark.
Signatures indicate what the person wants to show the world about themselves. They can be highly revealing. Signatures should be legible. Illegible signatures indicate that the writer does not care to communicate who they are. Occasionally a person will write with only a long line or scribbles thinking that this will make it very hard for anyone to forge their signature.