Shorter, cheaper flights. Toronto to Orlando is 2.5 hrs I think?
The only European airport that does this is Dublin, last I checked.
If you fly from London to Manchester, what happens when the plane lands? You get up and walk away, right? If you fly from London to Dubai, wouldn’t it be nice if you could just get up and walk away once the plane lands? Aren’t you tired of waiting in lines and ready to just fall into your hotel bed? Canadians can do that if they are flying to Miami or Los Angeles, but not Cabo San Lucas.
You could do this too if you flew from Dublin to New York, but not if you chose London to New York or Paris to New York.
However, as others have pointed out, this isn't what pre-clearance is. It's more akin to clearing French passport control and customs in Dover or rather than Calais, and can make the trip substantially shorter than it might be otherwise.
When I still lived in the UK, I found it quicker and more pleasant to fly from Heathrow (or Bristol) to Dublin and then on to the US having cleared immigration during the layover. It's also quite a bit cheaper when you fly from the UK in a premium cabin since Ireland doesn't have the egregious air passenger duties that the UK does.