Provided you do not have any accounts on your machine that do not require a password to log in, a password certainly provides an added layer of security as far as someone simply picking up your computer and being able to use it if it were left somewhere.
When it comes down to it, protection of actual physical access to your computer (or smartphone, or other device) is your absolute best line of defense against unauthorized use.
All security measures deal with making it more difficult for the unauthorized to gain access to your information if they have somehow gained access to your hardware (whether physically or remotely). There has yet to be a security protocol invented that cannot be broken into by someone.
User account control is more focused on what a user authorized to use the computer can and cannot do as far as tweaking the system itself.