Some carpets get really dirty from repeated walking in the same area like hallways and staircases. There are no other ways to walk through the area so you start to see a traffic lane developing. Soil starts to appear and slowly get darker and darker. In the pictures above the resident tried to clean some of the darker areas with some carpet cleaner which left the carpet with a blotchy appearance.
A good carpet cleaning will take care of the problem and remove the soil and the blotchy appearance. Traffic lanes can be difficult to keep clean and will eventually come back because there is nowhere else to walk.
In Tampa FL you have a lot of traffic lane soil. A lot of homes in Tampa have tile floors adjacent to carpeted areas and traffic lanes are sure to develop at entry points into carpeted rooms. If you get a carpet cleaning on a regular basis this won't be much of a problem. The longer it goes the without being cleaned the tougher it will be to get it cleaned down the road.