"dear" is a term of endearment. It means the person thinks of you as a friend, someone close to them, or simply they want to sound less formal. I hope that answers your question :)

Sometimes older ladies will say this to just about everyone; it's a way of being friendly and showing you care about people.

I don't know how often it is used in other countries, but in America the word is mostly used to address your spouse or your child. It was used to address a wider variety of people in the past, so older folks may use it. The only other use I can think of is you address someone in a letter (postage or email).