Then another person, friend or family, can come on and see and use your PC without any restrictions.
It is advised to have a password set. That way even if you get hacked they still have to get by your password.
I am the only one that uses my PC. But yet i have it password protected. Not even my wife knows the password. This is for later on when my daughter gets older and starts to use a PC. That way i know my PC;s are protected and she cant get on to do whatever she wants without my consent.