Getting a document notarized means that you have someone who is registered as a notary witness that you or another person signed the document on a given date and time. Notarization can make a document more formal, and sometimes it's required for a document to be processed by a court or agency. Even when notarization isn't required, it can be a good idea.
One of the top reasons to notarize signatures on contracts is that it helps keep someone from later claiming they didn't sign the document. Individuals have been known to make such claims in court when disputing a contract or as a way to get out of a contract. Other claims used to back out of a contract might include that a person didn't realize they were signing the agreement or that they were coerced into the agreement. A notary's witness and signature and certification help dispell such claims.
In fact, in many courts, notarization means a signature is automatically considered valid. In other cases, you might have to spend valuable time and resources authenticating a signature before you can move forward with any disputes related to a contract or signed document.
If you're working with an attorney to vet or create contract documents, then seeking a third-party notary is usually not necessary. Many lawyers can also notarize or witness documents or they have someone on staff who is able to do so.
Ensuring signatures on your contracts are valid and above reproach is just one step to protecting yourself from costly contract disputes. Working with a professional to ensure a contract is as strong as possible helps keep litigation at bay, but if you do run into a contract dispute, getting a professional on the scene as soon as possible can likely reduce the headache involved.
Source: Entrepreneur, "Avoiding Contract Disputes," Chris Kelleher, accessed Dec. 31, 2015