for plastisol transfer, I never really use it. No need.
only if the inks need to be protected from the iron's direct heat/contact, but the release sheet does that.
i do use it sometimes after the transfer have already been printed on the shirts to give it a differenat texture/look (you can add alot to plastisol transfer inks like glitters as you print your transfer)..... or to fix up screw ups.
for freestyle projects etc. it comes in handy. Again to protect what has been printed already.