Polishing follows the cleaning
The cleaning steps leave the tooth surfaces roughened; unsmoothed, they would quickly be colonised again by bacteria and deposits.
Cleaning leaves traces. Instruments and the powder jet do take the deposits away, but they roughen the surface slightly – and on rough spots bacteria settle again faster than on smooth ones.
That is why polishing follows
With soft cups, small brushes and a fine paste the teeth are smoothed, along the gum line and between the teeth as well. This is not a cosmetic step but the part of the appointment that decides how long the result lasts.
A fluoride varnish usually rounds it off. It hardens the enamel and protects the freshly cleaned surfaces during the first few days, when they are particularly sensitive.
Why polishing is more than the final touch is explained by the Prophylaxe-Berater:Polishing the teeth