As a tower controller, I have to keep departing traffic on my frequency until I know there's no confliction with anything going around from short final behind it. (I guess this is particularly relevant when dual runway ops are in progress at Manchester and Heathrow). In the event of having to issue avoiding action turns, it's much better to not have one crew in between frequencies (or having to coordinate avoiding action via a telephone line which takes time). Plus, of course, better situation awareness for both crews involved.
I remember one pilot giving a presentation to ATC and mentioning our 'magic camera' on the flight deck, which enables ATC to see whenever the crew is busy and hence to speak to them at exactly the wrong moment! We try to issue frequency change instructions at a sensible point on climbout (one A320 operator let us know that after 2500' is a good time, for example; fast-climbing positioning flights might want an early change to enable continuous climb, etc), so if any crews are reading this, any input on what helps you would be gratefully received!