It's used to show respect. Military personnel say "Yes, sir" "Yes, ma'am" "No, sir" or "No ma'am" to their superiors. Likewise, employees in businesses may address senior executives that way. Where I grew up in the southern part of the US, it's also traditional for children to address adults with "sir" and "ma'am."

In the UK, it is similar to the US actually. Children at school address make teachers as sir. Staff in shops and restaurants address male customers as sir. It's also used on the military. However, it's not really used respectfully to address older men in general. When used as a title of the nobility, you say Sir Name, e.g. Sir John.

I agree with Jonathan. It is common for people in the military to use this word, or sometimes the police. In normal life, it is rare to use it, unless you are making a joke. Although, sometimes, if I am trying to get the attention of a man that I don't know, I will say: "Excuse me, sir!" Estoy de acuerdo con jonathan. Es común que las personas en el ejército usen esta palabra, oa veces la policía. En la vida normal, es raro usarlo, a menos que estés haciendo una broma. Aunque, a veces, si estoy tratando de llamar la atención de un hombre que no conozco, le digo: "Excuse me, sir!"