Computers get dirty. They are basically dust sucking machines with their fans drawing dust in all day long. The computer's CPU in particular gets filled with dust. The problem arises that the dust collects on computer components and then attracts water so that the computer can get corroded. In time this can completely destroy the computer. Tablets tend not to have this problem, but laptops and desktop computers do.
Whenever a computer comes into our workshop we not only give the software a clean up but a physical clean too. Removing the dust regularly can substantially increase the lifetime of a laptop or desktop. We use various tools for this but a significant part of the process involves blowing compressed air through the computer. Traditionally, this has come in the form of cans of compressed air. We would buy them in boxes of 6, and end up going through many cans of air in a month.