A digital certificate is imporatant for a digital signature because it provides the public key that can be used to validate the private key that is associated with a digital signature. Digital certificates make it possible for digital signatures to be used as a way to authenticate digital information.
A digital signature or ID is more commonly known as a digital certificate. To digitally sign an Office document, you must have a current (not expired) digital certificate. Digital certificates are typically issued by a certificate authority (CA), which is a trusted third-party entity that issues digital certificates for use by other parties.