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
A flat-top stopper must be laid with its flat sides on the table to ensure stability and prevent it from rolling or tipping over. This orientation maximizes its contact with the surface, providing a secure base that minimizes the risk of accidental spills or breakage. Additionally, positioning the stopper this way can facilitate easier access to its contents when needed.
There are 2 completely flat sides on every cylinder.
square
A triangle has three sides !
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