It means that if you do get refused entry you get to go home that day rather than being put in a holding cell then sent back home at your own expense

And whilst it would be nice to do pre-clearance at London airports - have you considered the problems it would create for people connecting through London in order to go to the US. It would increase the minimum connection time by at least an hour.

And how do you handle flights that go from country A to the US via country B where you will have passengers who are just going between countries A & B