It's common knowledge that if somebody has physical access to your machine they can do whatever they want with it1.
So why do we always lock our computers? If somebody has physical access to my computer, it doesn't really matter if it's locked or not. They can either boot a live CD and reset my password or read my files (if it's not encrypted), or perform a cold boot attack to get my encryption keys from memory (if it is encrypted).
What's the point of locking a computer besides keeping the average coworker from messing with your stuff? Does it provide any real security benefit, or is it just a convenience to deter untrained people?
1. unless the computer has been off for a while and you're using full-disk encryption