Why sign terms?
Digitally signing of the terms posted on your site helps assure your existing and potential customers that you take these terms seriously, at the same time it helps avoid conflicts by protecting integrity and timestamping of the document, timestamping allows to prove that the document existed at a certain point in time.
Consider the following situation. You offer a service that you sell online. When your customers subscribe to the service, they have to agree to your terms. The terms include a provision that e.g. you can stop the service for the customer in certain circumstances. A customer subscribes to your service and after some time you do stop the service referring to this provision. The customer claims that the provision didn't exist when he subscribed and you added it later.
Solution. Sign the terms. When you digitally sign the terms, a timestamp is added to your signature, which allows to prove that the terms in their present version, including the disputed provision, did exist before the customer subscribed. If you had edited them after signing, the signature wouldn't have verified. According to E-SIGN Act, this electronic signature has same legal effect as the conventional paper-world signature, and it can be considered as proof in court. Also, when your potential customer sees "Genuine Document" seal on your terms, he can be sure that you won't be able to silently edit the terms in the future, because that won't go unnoticed.