There are so many reasons why a clutch disc can fail. First of all, they are coated with a friction material similar to that of brake pads. In time, it will surely wear out. Premature failure is usually the result of an oil or transmission fluid leak. That will corrode or ruin the friction material on the disc. Another likely condition that could cause the clutch disc to wear down prematurely would be because of user error. If the driver rides the clutch (meaning slightly resting their foot on the depressed clutch pedal), then it's likely that the disc surface is continuously slipping off the rotating flywheel or pressure plate; rubbing off friction material. Evidence of this will be a worn out release bearing along with a shiny, smooth disc surface. Without a sticky clutch disc, you will not be able to shift properly and the driving experience will be unpleasant and inefficient. You may notice that your engine's RPMs will increase after a shift but your car will not accelerate. These sudden and unexpectedly high revolutions indicate that the disc is not sticking to the flywheel.
Manual transmissions require constant engagement with the driver, and their ability to engage the engine to the transmission depends on a properly working clutch. When the clutch pedal is not depressed, the clutch disc (which is joined to the transmission input shaft) spins along with the flywheel and pressure plate. As the clutch pedal is depressed, a fork pushes the release bearing into the pressure plate springs and that relieves pressure on the disc. With the disc now floating in the space between the pressure plate and the flywheel, there is no connection between the engine and the transmission. This small window allows the driver to change gears. Then the driver releases the clutch pedal, bringing the clutch disc back into contact with the flywheel and accelerating into the new gear.
Replacing the clutch disc is usually done on a regular basis as a preventative measure. Different manufacturers have different recommended service intervals, but they usually last somewhere between 60,000 and 90,000 miles if driven properly. In order to access the clutch, you have to either lift the engine out of the engine bay or drop the transmission. Either way, it requires a lot of time and expertise to be done properly. The basics are: