I never understood the need for it. I've been in situations where I, or a player in my group, is in a blind spot on a hole, and got turned around enough to not know which direction the basket was. It's kinda time consuming when the thrower walks up, visualizes the target, walks back, loses his place, walks back up, walks back, etc.
In such cases, by rule, setting a bag or a towel on the ground for aiming purposes is illegal. But if another player lines himself up between the thrower and the target to point the thrower in the right direction, then ducks out of the way before the throw, that's fine. Obviously it's not physically placing an object but it's done in the same spirit as placing the object, so why shouldn't it be treated the same way?
Absent a rule disallowing it, players that place an object as a visual aid run the risk of hitting the object with their throw, which introduces potential interference penalties. That seems a reasonable enough deterrent to me.