Alfred, think of QWI as equivalent to a PWO in the RN and you won't be far off.
QWI isn't compulsory for rapid advancement in the RAF, but if you look at Sqn bosses and Stn Cdrs the vast majority, in the FJ world, are QWIs and those that don't get FJ sqns/stns often get the other sqns/stns (Valley, Waddo..).