Common phrase used as an answer to "Why do you ask?"
Usual Meaning: I was interested in your answer and will base my future responses around said answer.
Very similar to "Just asking"
Guy: Are you single?
Girl: Yes. Why do you ask?
Guy: No reason I was just curious.
Girl: Hey what are you doing tonight?
Guy: Not much, why whats up?
Girl: I was just curious. (Will generally follow up with girly phrase implying she wants him to hang out with her.)