Towers are almost designed to deposit this dust as the air enters, swirls around, drops some of the laden dust, and moves out of your computer case. This is happening all the time your computer is on so even in the cleanest home or work place it becomes a problem.
It's a problem as the dust stops the cooling fans and heat sinks from doing their work efficiently, your computer gets hotter.
At some point problems can start manifesting as the temperatures in the tower as a whole or on individual parts becomes too hot.
Cleaning the dust from your computer is important, but it also must be done carefully as breaking components is a real possibility and we cannot forget the dangers of static electricity.
Cleaning computers is one of the many services that Cameron Computers offers.
Below is an extreme case of a dusty computer, and a link to many more.
