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.
We apply the term mesa to a flat-topped mountain.
plateau
Butte
A butte is a flat-topped landform feature found in arid regions that is created by the erosion of a mesa. It is typically narrower and taller than a mesa, with steep sides and a flat, table-like top.
A high flat-topped land form with cliff-like sides that is larger than a butte is a "MESA."
Mesas are broad flat-topped landforms with steep sides. They are commonly found in arid regions and are formed by erosion of sedimentary rocks.
Yellowstone national park
DUNE