The first thing to note is that flight schedules are typically (supposed to be) valid for long times. While (to the best of my knowledge) there is no common timetable change date as European railways typically have it, the same flight will be scheduled to depart and arrive at the same time over months. However, flying conditions—which depend mostly on weather—change daily. One day you will have favourable winds, the next you will have mediocre winds and the third you will be up against the jetstream. This is why the transatlantic corridors (and probably other flight paths although I would consider them less ‘famous’) are different routes every day.
The timetable time is then supposed to capture something like a 95 % punctuality rate (meaning in 95 % of cases a punctual landing can be expected)—but the airline will know on the night before what weather conditions and thus what actual time to expect and can submit that to air traffic control. Therefore, even though your landing might be scheduled for 8 a.m., the airport might still expect you at 7.30 a.m. on the 35th May just because of the favourable winds. This will seem like an early landing, but it was expected and the slot was ready.
I recently read in an article about Munich airport that long-distance flights (one to Beijing was singled out) only depart if there is a slot ready for the entire trip from take-off to landing—whether this is generally true, only for Munich, only for Beijing or a special case I do not know.
Of course, there is a second option: maybe there were actually unexpected conditions that allowed you to be faster than expected and arrive at your destination 15 minutes before you thought you would. Obviously, as soon as the aeroplane is in range, it will contact the necessary traffic controllers to state it is there. In airports such as Heathrow, they’ll probably never be tempted to land you early because they are already dealing with a queue all the time. But in not-so-busy airports they might notice another flight being delayed, a slot for you being open—and then there is practically no reason not to get you out of the air as soon as possible. It has a number of advantages:
- ATC doesn’t need to keep you in hold or indeed concentrate on you at all, you’re safely on the ground
- any emergencies that might turn up in the 15 minutes you are early can be dealt with rapidly on the ground without needing to land first
- no sudden technical difficulties, bird strikes or other shenanigans can cause you to fall out of the air, because you’re already on the ground
- in the case that you need to abort a landing and initiate a go-around you still have ample time to be on time (and probably your originally intended slot)
- it allows later planes to come in earlier, hopefully catching up a delay as you’re already on the ground
- from the airline’s point of view, the plane is not burning fuel and that means big bucks.
Of course, this has a caveat: on the ground, you need some place to go. Probably that will be your assigned gate, usually for organisational reasons. However you might be so early that the previous plane is still there or is just leaving so you can’t park there just yet. Well, any decently sized airport where gate capacity might be a problem likely also has a lot of taxiway that you can intermediately wait at. It’s definitely cheaper and easier for everyone involved to just sit around on the taxiway to wait rather than circling in a holding pattern—see the reasons above. So the minor inconvenience of keeping the passengers seated for an extended period while the intended gate is being freed is most certainly the better choice.