This is a way to introduce a request. It makes the question sound really polite:

Hey Jen, I was wondering if I could borrow your textbook over the weekend.

I was wondering if you'd like to go out for coffee some time.

You say "I was wondering if..." even if you're still thinking about this question now. There's no grammatical reason for using the past tense ("was wondering"). That's just how the expression is said.