An X is a legal signature. Signatures are really only useful as an indicator that you agreed to something in entirety, it clears up legal grey areas. So, when

An X is a legal signature. Signatures are really only useful as an indicator that you agreed to something in entirety, it clears up legal grey areas. So, when a judge asks "did you agree to this?", you can't say "I agreed to X not Y". Instead they can say "is this your signature?" and the intent is clear. It's really just a symbol of total agreement.