Computers as they work eventually build up dust inside them. For Desktop computers, if not cleaned often can build up into thicker dust particles sticking to computer parts such as the fan. A computer should be cleaned out often for multiple reasons.
1: A computer that has too much dust blocking off the ventilation can start to get hot and maybe overheat and shutdown.
2: A clean computer is a happy computer. Think of the computer as yourself when you breathe you want to breathe fresh air, well the same goes for the computer it needs to be clear of all dust.
A good solution to get rid of dust is to use a can of compressed air such as DustOFF. This works for other things as well such as keyboard, laptops and more.