1) in the 20th Century why was that necessary?
Simply for the reason the 20th Century saw the issuance of imperforate stamps. The US Farley Special Issues for example. As to other imperforate stamps, there were reasons, most production and demand related, for imperforate material.
Even now in the 21st Century I go to a post office and receive stamps separated by scissors by the clerk so the edges of the stamps are not damaged. For example the US 5297 Statue of Freedom $5 is produced with a strip of non-stamp dividing paper between the stamps. Cutting along that paper produces a single with no damage to the stamp itself.
From a multiple:
Seek out writings on the subject of stamp separation by John Hotchner.
His exhibit on the matter may be available as well. Perhaps someone will add a link.