Several reasons: 1) bacteria and molds can grow under the bands and cause the seal to fail or the food to be contaminated, 2) it facilitates proper washing/disinfecting of the filled jars prior to shelf storage, 3) it makes weak or failed seals very obvious to you when jars are removed from the shelf for use since if the bands were left on the loose lid may not be noticed, 4) bands can rust while on the jars making them impossible to open, and 5) it allows for contaminated jars to just pop their top rather than exploding.

Stacking canned goods is easily done if you wish to do it. Many prefer not to. Even with the bands removed. You wouldn't want them stacked directly on top of each other anyway so you have to distribute the weight of the second row by putting a layer of thin plywood or cardboard on top the lower level and then stack the second level on it.

Hope this answers your questions.

Dave