The point of an Advance Stop Line is to give an area for cycles at a light-controlled junction at the front of the waiting queue of traffic. This is for safety reasons: it makes cycles more visible and less likely to be vulnerable to dangerous overtakes.
For example, here's one in Trafalgar Square:
And another one:
One more here:
Now the observant reader may have noticed a pattern emerging. I didn't have to wait long to take these pictures. In fact I would have had to wait a while to get a picture of the ASL without a motor vehicle in ito. Failing to observe the stop line is an endorsable offence with a £60 penalty. So there's £420 worth of fines just in these pictures. Trafalgar Square is a very busy and dangerous junction, a gyratory system with multiple lanes and exits. Exactly the kind of junction where ASLs are needed, especially given the relatively high volume of cycle traffic going through it. But I've never seen the police enforcing the ASLs here - even though catching offenders here would be like shooting fish in a barrel. I have seen a police car encroaching over the ASL however.