To come in handy means to be useful and often times in an unexpected way. In general, anything can come in handy. It could be your umbrella, especially if you almost didn't take it with you and it suddenly rained. It could be your college degree if, for example, you found a temporary, lucrative position that needed someone with a degree and fast. It could also be the little bit of Spanish you learned in high school if you found yourself in a Spanish neighborhood and were able to ask directions successfully. When you use something that you already have, you can say that that thing came in handy.
A: I hear John kept you waiting for 30 minutes last night.
B: Tell me about it. It was so cold. I'm glad I had my thick scarf.
A: The one I gave you? That's great! I'm glad it came in handy.
B: Oh yeah. I would've been miserable without it. So how was the dinner with your boss?
A: It went fine. There were some French people there, so my two years of college French actually came in handy. It was great.
lucrative - something that brings in a lot of money
Tell me about it - definitely; I agree (see the Oct. 8 blog)
Alright folks, the next time you use something that you already have, you can use this expression and say, "It came in handy."