The bootsector allowed an automatic startup (vs tapes) of games like in consoles with cartridges (of course a lot of games came also out as cartridge for computers). Put it in and switch on computer and play. Of course the bootsectorviruses were a sideeffect of this innovation. 

Floppy disc or magnetic disc are faster and mor flexible than tapes. you can load and store autonom (no start and stop) and not linear, you can store them here or there. but of course also expensiver (you cant anymore use a (music) tapedrive.

r and c discussed if a bootsector virus is possible (in 512 bytes). than c went home and coded in2 days one, gave a infected disc to r with software. legend: from this disc all sca viruses shall come from.

story was told in a discussion around 2013 by c.