A computer's biggest enemy is heat. The hotter a computer is, the less efficiently it runs, the worse its performance, and the shorter its lifespan. Computers rely on positive ventilation for cooling, and the biggest obstacle to that is dust.

Unless you're working in a true clean room, then even the cleanest server room will have dust, lint, fiber and other tiny bits in the air. Overtime they're drawn into a computer by its cooling fans, and build up on the fans themselves, on heat sinks, and elsewhere in the machine. For a non-critical machine, it's a good idea to clean it out once a month. Just take some canned air and blow all the dust out, using a soft brush to gently loosen any that won't blow off. Even a mission critical server can benefit greatly from periodic cleaning, but you'll have to decide yourself when you can afford to take it offline and for how long.