Usually there's an obstacle intruding into the approach gradient. The solution is usually to 'jack up' the approach path to provide a certain, defined amount of clearance above the most critical obstacle.
The result of raising the entire approach path is that it intercepts the ground ie the runway, further away - so that's where the beginning of the useable part of the landing runway (in that direction) is marked.
If this were not done, pilots would aim for the marked threshold at the beginning of the tarmac & be at risk of colliding with the obstacle.
Each runway useable in both directions has 4 separate obstacle clear gradients calculated for it:
1. Obstacles after take-off in one direction.
2. Obstacles on the approach path in that direction
3. Obstacles after take-off in the opposite direction.
4. Obstacles on the approach path in this opposite direction.
With the various buffers that are applied this determines the minimum gradient that must be achieved to clear the obstacles safely.
There are limits on just how steep an approach or departure path can be - hence having to move the threshold or reduce the available length for take-off.
Some runways use special procedures to circumvent particularly awkward obstacle clearance problems eg London Docklands has an unusually steep approach - so much so that pilots AND aircraft have to be certified to operate into the airport.
NB: I think London Docklands also has an element of noise abatement in it?
Other runways may have a kink or bend in approach path eg Kai Tak, or have the final approach be slightly offset.
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Any errors are entirely my own but at least they are mine.....all mine......mwah ha ha ha......
Oy...who are you chaps? Why the funny white coats? Can I have one?
LET GO OF ME!!!