Hello m:

Contracts are valid even WITHOUT a witness signature.

excon

Depends.

In many states, for example, a will needs witnesses.

But usually it's just a matter of formality and proof. A place is left for a witness signature in case there is a witness handy. Such a person can testify later, if a question arises. It also tends to impress the parties to the document of the seriousness of it.

What was signed ? Some things may require a witness for the document to be accepted. But it is not required for most