Mylar bags are only a barrier if the seal is one. The usual improvised method is a household iron and a board, which does work, but it is inconsistent across a long seam and the failures are invisible: a bag that looks sealed, holds for a month, and has been slowly breathing the whole time.
An impulse sealer heats a wire for a set duration and gives the same seal every time. If you are putting away more than a handful of bags it removes the one step in the whole process that silently undoes the rest of it.
Get one wide enough for the bags you are actually using. A 200 mm sealer on a 250 mm bag means two overlapping presses and a weak point in the middle, which is exactly where a seal fails.
It is not needed for a vacuum sealer setup, which does its own sealing, and the two solve overlapping problems. A vacuum sealer is better for portioning things you will open within months. Mylar and an impulse sealer are better for putting a sack of rice away for years.
Worth a check: fold the top of the bag over and seal again below the first line. Two seams cost a second and remove the most common failure.