come in useful
To be especially helpful, often in a particular setting or situation. It was a lot colder out than I thought it would be, so your jacket really came in useful! That flashlight comes in useful when I'm trying to find something in my car at night.
come in useful
to be useful. Your report has come in useful a number of times.
ˌcome in ˈhandy/ˈuseful
(informal) be useful when needed: The money my aunt gave me will come in handy to pay for my music lessons.