make (oneself) useful
To do something that is actively helpful or of assistance to others. You know, instead of just sitting there watching TV, you could make yourself useful and take out the garbage. Towards the end of the party, I decided to make myself useful and wash the dishes.
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.