So that whatever leftover gunk happens to be on the table doesn't get all over the part of the stopper that goes into the bottle, contaminating the contents.
ziggurat
There are 2 completely flat sides on every cylinder.
square
A triangle has three sides !
Faces are sides but sides (as in sides of a square) need not be faces.
So they don't roll.
So that you don't contaminate the stopper with anything on the table. Also, if you lay it on it's side, there's a possibility that it rolls off the table and breaks.
plateau
a butte or a mesa.
DUNE
plateau
Butte
A high flat-topped land form with cliff-like sides that is larger than a butte is a "MESA."
Yellowstone national park
A mesa.
Table, butte
DUNE