It's so that you can be asked "Did you sign this?" and you would be lying if you said no. And so that if someone else signs it in your name they are leaving evidence of fraud.

If someone else signs for you and you say it WAS you, I don't think the signature is going to be disputed. You can always say you just decided to sign it weird for some reason. In the opposite case, the forger is going to need to argue that you signed it weird, while you are going to say that you didn't sign it at all.

But that's how I think things are supposed to be.

I live in a strange country where signatures actually are compared by clerks and will literally reject your second signature if it doesn't look enough like the first, even when you signed both right in front of them.