Airlines, trucking companies, and other large organizations devote much more time, money, and effort to cleaning and preventative maintenance than they do to repairs. That's because cleaning and regular system maintenance repay their costs many times over by reducing the frequency and cost of repairs. It's almost always cheaper to prevent something from breaking than it is to fix it after it's broken. The same is true for PCs.
Dirt is the main enemy of PCs. Dirt blocks air flow, causing the system to run hotter and less reliably. Dirt acts as thermal insulation, causing components to overheat and thereby shortening their service lives. Dirt causes fans to run faster (and louder) as they attempt to keep the system cool. Dirt worms its way into connectors, increasing electrical resistance and reducing reliability. Dirt corrodes contact surfaces. Dirt is nasty stuff.
Computers become dirty as a natural part of running. Fans suck dust, pet hair, and other contaminants into the case, where they rest on every surface. Even in clean rooms, operating theaters, and other very clean environments, a PC will eventually become dirty. If there's any dust in the air at all, the system fans will suck it in and deposit it inside the case, where it will become a problem sooner or later.
The severity of the problem depends on the environment. Industrial environments are often filthy, so much so that ...