There are a number of reasons to switch the pole screws in your humbuckers. The screws carry the magnetic field from the magnet - sitting underneath the bobbins, next to the base of the screws, to the string above. As you're probably aware, different magnets cause tonal changes in humbuckers - and the dimensions of the screws that carry the magnetism to the string do too. A different sized screw will form a different magnetic field around the string, meaning that it will be magnetised differently.
A shorter screw will provide a sharper, more articulate tone, as the magnetic field is more focused. A longer screw will provide a looser, vocal quality. Shorter screws are better for heavier styles of music, whilst longer ones are great for softer styles.
These screws come in 2 sizes - 16mm (standard humbucker pole piece length), and 13mm (shorter than normal).