Why change clutch fluid?

[Vyse]

Can anyone give me an explanation as to why a clutch fluid is change is required after x number of miles? I know clutch and brake fluid is hygroscopic but how does this then relate to the performance of the hydraulic systems?

Thanks

[Vyse]

With my car I sometimes experience grinding when changing gear. Its 4 years old and I assume the brake/clutch fluid hasn't ever been changed. Although looking in the reservoir the colour is black just slightly brown.

Can I assume a fluid change can solve the grinding? With moisture in the system, I am maybe not getting full compression/action of the slave cylinder hence the clutch not disengaging fully.

Gearbox oil has also been changed but hasnt solved the grinding.