You may find Rose Brand's
interactive curtain tutorial educational. You can see by the way they open that different styles lend themselves to different uses.
The main curtain has a lot of names, the common
bit being the word "main". In most places I have worked they
call it the main
tab even though it wasn't ever actually rigged as a
tab curtain.
Drapes get selected for lots of reasons, aesthetics and physical limitations of the
venue being the 2 that spring to mind.
In my
theatre we don't have a main drape. If we did, it would likely be a
braille or venetian because we would not want to sacrifice the
wing space at the
proscenium required by a traveller, or any other drape that gathers into the wings. We lack the height for a full
fly tower so that limits the our ability to
fly out a
guillotine without getting into some sort of
tripping arrangement that starts to encroach on the LX1 position.
Some venues change out their main drape to suit the show that's on
stage. It must be nice to have that kind of money.