@curious101 asked: "Why are they not necessary when vacuum sealing"
A good vacuum seal(er) will remove ALL the gases(most importantly oxygen) in a container. This limits the amount of molecular activity that can take place in the container, such as drying out, decay, etc. That is why vegetables and meat will stay fresh for a year or two(or longer in a freezer). The Boveda pack contains some air and water and therefore has hydrogen, oxygen, and other gases in it so you don't have a true vacuum in the container. While I don't think it will affect it much, still, something that costs a couple dollars is being used where it is not needed.
In summary, you spend money on a vacuum sealer, you take the time to remove gases like hydrogen and oxygen from the container, then you put a Boveda pack that contains hydrogen and oxygen back into the container. That is why Boveda packs are not necessary when vacuum sealing.
Longball