Why do people put passcodes on their iPods or iPhones?

My girlfriend has an iPod touch and my friends daughter has an iPhone and they put passcodes on them. I have an iPhone and I don't need one on mine. My friends daughter has a little brother and I understand that's why she has it on hers but my girlfriend doesn't have any brothers or sisters so there's so reason for her to have it on her iPod. I don't get why they put them on there. Why do they have passcodes on their iPods and iPhone?

If it gets stolen, nobody will be able to use the phone. They may have it for privacy, noone will be able to go through the phone, mostly privacy.

Some people do it because they have things they don't want others to see. Other people do it just incase someone tries to steal it they can't get into it

To keep things private and offer a little more protection if it gets lost or stolen, than just leave no pass code.