Is there any reason to keep materials under completed buildings?
We usually return the cards used as materials to the box when the building is completed, but I've seen some who keep them under the building. To my (limited) knowledge there's no rule or card that needs them there, and they do clutter the table, so... what's the point?
If everyone can easily reach the box, there's no reason not to put the cards there. But if the box is off the table, or on a corner that not everyone can reach, it's easier just to put the cards under the completed building.