... you meet a lot of people... And you might not be interested in all of them.

I know the thing about being clear and direct. But I'm NOT talking about one on one dates or online dating here. I'm talking about meeting a lot of people in person and in social events and social occasions. Sometimes you meet people in droves and so you flirt as a part of your personality to generate more interest in others. This may then lead to people being attracted to you more and those who are attracted to you will escalate it to actual flirting. You are basically filtering out those who find you attractive so you don't have to waste time with those who don't like you at all.