parfocalizing eyepieces, rings w/helical focusers

Why is parfocalizing recommended w/helical focusers? Guess I dont understand what it means. Are rings used to convert no parfocalizing eps?

Parafocalizing means adding spacers (metal rings or O-rings, etc.) to the barrels of your eyepieces so that they will focus at roughly the same point. It is recommended when you use a helical focuser since it can take a lot of turns (and time) to get a helical focuser to move far enough to acheive focus.

Joplin