Most carpet does not get dirty in a way you notice day to day. It goes grey a little at a time, and because you walk on it every morning your eye adjusts right along with it. The moment people actually see it is when half the room is done and the other half is not.
That line down the middle is the same carpet, same light, same day. Nothing was replaced.
Traffic lanes
The darker paths running between doorways are where the pile has been walked flat and packed with soil. Sand and grit work down to the base of the fiber, and every step grinds it against the twist. Caught early this cleans up completely. Left for years it dulls the way the fiber reflects light, and that part does not come back.
Filtration soil
The grey lines along baseboards and under doors are not spills. They are airborne soot and dust pulled through the carpet edge as air moves through the room, and they need a different treatment than a traffic lane does.
What you should expect
A cleaning will not make worn carpet new. What it does is pull out the soil that is still loose enough to lift, restore the pile, and stop the grinding that causes permanent wear. On carpet that has been maintained, the difference is the one in the photo above.