This allows a malicious server to send you different content when you pipe to bash than it sends your browser (or even to a script), hiding the attack from peop

This allows a malicious server to send you different content when you pipe to bash than it sends your browser (or even to a script), hiding the attack from people that give the script at least a cursory look. It's fundamentally not much worse than curl > file.sh && sh ./file.sh, but even if the victim notices something odd, all the oddity is invisible.