Traffic wear patterns aren't caused by soil; at least not in the way most people think. As soil accumulates in your carpet, the sharp edges of that soil, scratch the fibers creating a dull appearance.
This Damage is Irreversible!
Nightly vacuuming goes a long way to prevent build-up, but vacuuming alone can't get it done. At least twice per year you should have your carpets professionally steam cleaned to protect your investment and keep your business looking nice.
If you keep your office doors close and notice a line forming, or along the edges of a room, you will want to consider quarterly cleaning. These darkened areas are known as filtration soiling and are created when air flow causes the fibers of your carpet to act as a filter.