contemplativeOnce upon a time stamps were perforated sheets of gummed paper. They were perforated in order to let you pull them apart more easily before licking them.
Now many stamps come in the form of stickers which don't require licking or dampening to stick to envelopes. However, they are still the same shape with edges that give the impression of something that has been separated from a perforated sheet.
Why?
Also, I suspect they just put the little frilly parts on to make them look nice, since that's the sort of thing people like that do.