The laws in California have specific requirements when it comes to signing estate planning documents. Some require witnesses, whereas others require notarization or other formalities.
For example, certain types of Wills need to be signed by the Testator (creator of the Will) and the signature must be witnessed by at least 2 other people, all of whom should sign the Will. If a document needs to be recorded at the Recorder's Office, then the document must be notarized.
These laws are enacted for your protection. They help ensure that you understand the seriousness of your actions. The formalities may also have the effect of preventing others from forging your name and/or giving notice to the public as to the ownership of your assets, such as with recording of deeds at the Recorder's Office.
Out-of-State Estate Planning Documents
For people moving to California that already have an existing estate plan, a concern is whether those documents are effective under California law. As a general rule, if the documents followed the formalities of the state where you are moving from, then it should be valid under California law. For a specific statute involving a Will, see California Probate Code Section 6113.