... because it's important to show all ajahs are equal in standing and toward the Amyrlin. The layout of the Hall shifts according to purely trivial matters, such as who gets there first and where she wishes the ajah to sit that day - there's no set and traditional positions for the Ajahs, it's precisely what this tradition seeks to avoid.

That's why the sisters use a cloth covering instead of painted benches or another permanent marking. They don't move the boxes or chairs around. the first Sitter from an Ajah to arrive claims a box, spread the Ajah's clothe on it and sits on her chair/bench
. The piece of clothe is probably installed in front of the chairs/benches.

It's really no biggie at all, no more trouble than putting on your shawl before going to a Sitting of the Hall...