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.