Why do so many English names end in -man, -ton, -son, -ly, -ing, etc. Becuase that is their lknguistic and cultural tradition.
In Polish the -ski is an adjectival ending and (as you probably remember from school) an adjective is a word that describes someone or something as being of, about, related to, descended from, associated with or connected to someone or something else.
A kowal is a blacksmith in Polish and the adjective kowalski means of, about. concerning or (in the case of a person) descended from the blacksmith. So the name Kowalski means little more than Smithson, although at times it was also applied to the blacksmith’s helper or apprentice. Even more commonly the -ski ending was toponymic indicating that a person was from a certain place: Brzeziński hailed from Brzezina (Birchville), Gwizdowski from Gwizdów (Whistleton) and Wi¶niewski from Wi¶niewo (Cherryville).
Names evolved from Jan z Dębowa (John of Oakville) to Jan Dębowski.
Incidentally, a similar but not identical process occurred in English, where the Middle English John of Oakville became simply John Oakville.
In general, the -ski is a high-class ending in Polish. More Polish nobles had ski-ending surnames than those with any other suffix.