Silicone is a treated parchment paper with a silicone coating on it. Teflon is a sheet made of teflon. Both are made to not stick to transfers, but its really a preference thing for most people. I do
dtg printing and use to do transfers, and I always used the silicone parchment paper. Alot of people also prefer the teflon sheet as they dont wear out. So its really a choice of what you prefer. The silicone parchment and the teflon sheet are both used for covering the transfer when pressing so it doesnt stick to the top platen of your press. So yes it is important, but the choice is really yours which you prefer
Hope this helps to explain things.