A good vacuum removes loose soil from the top of the pile. It does not remove what has been walked into the base of the fiber over several years: grit, skin cells, pollen, pet dander, and the sticky residue from spills that were blotted but never rinsed.
That residue is the part people underestimate. Sugar and protein from a spill stay tacky, which means every future footstep presses more dirt into the same spot. It is why a stain you cleaned six months ago comes back darker than the carpet around it.
Hot-water extraction works differently from vacuuming. The carpet is pre-treated so the soil releases from the fiber, the pile is agitated so the product reaches the base, and then hot water and suction pull the loosened material out of the carpet instead of moving it around.
Fiber matters. Wool, nylon, polyester, and olefin all respond differently, and using the wrong pH will either fail to clean or cause browning and shrinkage. This is the main reason a rented machine and a jug of supermarket solution often makes a carpet look worse a week later.
Drying matters too. A carpet that stays wet for two days will smell, because moisture in the padding is what causes odor, not the cleaning itself. Proper extraction pulls most of the water back out, and air movement handles the rest in a few hours.
For most homes, once a year is right, twice a year with pets or small children. Stairs and traffic lanes wear fastest, and cleaning them on schedule is considerably cheaper than replacing carpet early.