don I've seen that too and what I suspect is that the support is settled for their usual position with a normal chair and then at a contest/concert where you don't have other choices than a piano bench, which is also higher from the ground, they use a footstool to fill the gap. At least that's what I would do, other than that I don't see the point to use both except adding a point of instability
I do it. I got the "Ultimate Guitar Chair" but despite measuring my leg length as they asked, I must've got the wrong height anyways because it's just about 2-3" too tall. I'm 6'4 but I guess I did something wrong. In any event, I decided to use my "Embracing Guitar" guitar support (similar to a GuitarLift--the plexiglass board with suction cups) with the chair and it then required me to use my footstool on the lowest setting.