What are the ticks for when advancing skills?

In Troika! the numinous edition in chapter 11 Getting Better the rules suggest a tick next to the advanced skill on every successful skill roll (roll under) on the character sheet. After rolling for advancement of 3 skills (roll over) the rules state that all ticks have to be removed from the character sheets.

But what are ticks used for? How is the count of ticks relevant?