Basically, if you call 999 in an emergency, then you probably want the ambulance/whatever to reach you as quickly as possible. This will be a lot easier if they can see the number on your door, rather than driving slowly up and down the street, thinking "Ok, that's number 17, so number 23 will probably be three doors along from that". It's also probably easier to sort out the door number on a Saturday afternoon than it is at 3am when your flatmate isn't breathing. On a related note, if you do ever call 999 then it's useful to have someone standing outside (e.g. by the main road if you live in an estate) to flag down the emergency vehicle when it arrives. I did this quite a few times at school when we had fire engines out, and it's helpful to be wearing something that will show up at night.
It's also worth reading this article (by a person who answers 999 calls for ambulances):
When NOT to call an Ambulance
Basically, do call if it's an emergency, don't call if you just want a free taxi to the hospital. But that said, do call if you're not sure, as long as you'll be happy to take "no" for an answer. When I'm on SJA duty, I sometimes get people coming along to say things like "I've had a rash on my leg for a couple of weeks, do you think it's serious?", and I have to explain that they need to see a GP - I'm there to provide first aid for things that have just happened, not to solve long-term problems.