I'm wondering why alot of long-time para's have such low backrests on their chairs? Any advantage to this?

I'm a low quad with less than 6 months from injury and switched from a tall rigid backrest to a "normal" adjustable-tension-by-straps backrest and have been progressively lowering it. This has increased my trunk stability and it seems I have mid-back and abdominal muscle activation returning. I left rehab less than 2 months ago with a 19.5" Jay J3, now I'm down to 12" with my TiLite flexible, as soon as I'm comfortable, I lower it half inch.

Other than weight-savings is there any other reason some para's run as low as they can go? (like 5"???)