Sturdy enough to lean on, low enough to feel steady getting on and off.
Showering seated takes the risk out of standing on a wet, slippery floor, but a wobbly stool defeats the purpose. This one has a proper backrest to lean into, and adjustable-height legs finished with non-slip suction feet, so it stays planted through the whole shower.
Check the suction feet are seated flat on the shower floor before use, especially on textured tile where the seal can be weaker.