Runways are numbered based on compass heading. Why?
Does this help pilots in locating the runway, even in today's GPS age? Or are there some other technical reasons behind this?
There are two parts to the question:
The runways have to be identified somehow and the identification has to be directional. So the, rounded, heading offers itself and it has advantage that:
In the early days of aviation, magnetic compass was the only tool available for determining direction. It is simple and reliable and well tested in naval navigation. So magnetic headings became the obvious standard.
Later gyrocompass was introduced to overcome the limitation of magnetic compass that it is biased in turns and requires some time to stabilize. However gyrocompass precesses, so it has to be periodically corrected, which was, and is, still done by reference to the magnetic compass.
On the other hand GPS is still a rather recent technology. It is only precise enough to determine heading (or rather tracking) for last 15 years. It requires a complex electronic device that can fail in a myriad of ways and it is affected by many external factors including the state of ionosphere and the selective availability not being turned on again.
And not all aircraft have GPS anyway. So if you wanted to switch the ones that use GPS to true headings, there would still be some that would need to use the magnetic and it would be a mess. On the other hand when you have GPS, you have a computer already and it can contain and use the magnetic declination tables to give you magnetic heading as well.
The aircraft heading is measured with reference to the magnetic north. If the same convention is used for runways too, it will make it easy for the pilot to identify the runway and land as the aircraft's heading and the runway will be aligned.
For example, if the aircraft is directed to land in the runway 09, the aircraft heading will be 090° while landing.
Use of another system for naming will lead to unnecessary confusion. This convention is being used from long before GPS was available. Also,as already noted, a large number of aircraft flying today have no GPS and pilot should be able to land the aircraft even without it.