Tapes comes from the mainframe tapes and were popularised by music too. so often people used normal tapes-drives and for a zx81-games - there were even description how you had to connect the microphone-cable to the head-phone-jack and reverse. While loadibg a zx-81 game you hear the whole time the data coming in! so meta: sound was for people and maschines.analog digital. So there was no difference between an accoustic coppler and data from the tape. 

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.

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. 

sending information via snail mail on disks

most possible reason: you could several discs put into a letter.