I think the rack is to help get the jars out.
No, seriously.
My jars have always touched, fairly tightly. I think the problem, mentioned above, of having them not knock together when it boils could be an issue. I'm guessing they either need to not touch, or fit tightly enough that the boiling can't jiggle them aggressively. My mom canned her whole life with jars jammed in tightly together, and no worries. When all else fails, follow the blue book's instructions.
Old time cooks/canners like my mom and grandma canned in all kinds of jars––coffee jars, mayo jars––anything that would fit a standard lid and rim––and however many would fit in there, they shoved (carefully) in there. It was about getting the most for your heat and time. They didn't put
physical pressure on the sides of the jars by putting them in so tightly that they were stressed, but they definitely touched––on all sides cuz there was one (or two, depending on the jars) in the middle.
But I have never canned without a rack, and common sense tells me there should be a cushion of water beneath the jar. I don't think they should sit on the bottom of the canner. Plus, those hot jars are damn hard to get out of that water bath without a rack.