My immediate reaction is that they are not necessary. If you wrap whatever is going in the freezer well and use a good quality polybag, you should avoid freezer burn anyway. If you feel inclined to suck the air out, then buy a plastic straw, put it in the top of the polybag, hold the bag tightly and suck! Pull out the straw and use whatever you use to tie it up. But, be careful if you`re extracting the air out of a big bag (eg for a chicken) as it can make you feel a bit woosy!!!