Before the filament is retracted back to the park position behind the filament selector, there is a procedure called ramming (cooling and smashing the end of the filament). This is to produce a nice tip with no or a minimum amount of stringing behind. This strings can cause problems in some ways. They can make the FINDA misbehave, and when the filament is pushed back towards the hotend, it can fold back and make the tip stuck in the PTFE tube.
Also an amount of string pieces can clog the filament ways by time.
PLA can make some very fine tips, so unloading and reloading are for 99% with no issues.
PETG, it seems so, always produce kind of long strings, no matter what you set up. So is my experience. The reason is the different chemical construction and it's wide temperature range of a certain viscosity when hot.
So, a thick, almost non flexible tip could be cut of by the knife, but these string, almost as thin as spider web thread, can*t be cut.