On some documents they require a witness to sign. What is the point of this? Just added security that it's less likely 2 people would lie?
Generally a witness only witnesses the fact that X signed the document; they do not witness the content.
The primary purpose is to make it very hard for X to argue that they didn't sign. However, some documents (affidavits, statutory declarations, certain types of contracts etc) are required to be witnessed to be valid.