Vacuum packing removes the air from around your fillets before freezing, which is what stops freezer burn. Properly vac-packed fish holds its texture and flavour in the freezer for up to two years, compared to roughly six months in a standard bag or wrap. For deep water species like hapuka and bluenose (where one fish can fill a chilly bin)  it's the difference between eating well all year and quietly binning half of it in spring.