It's been useful to boot into WinXP to play AoE or WoW, or Mac OS 10.9 to run p0sixspwn to jailbreak an iPhone 4S. The laptop has a 2 TB drive, and a lot of legacy software just in case I need to open some obscure file (e.g. AppleWorks).
I regularly find old laptops in the trash, and friends like it very much when I repair them and give them away. Some only boot Win10, others only Win7 (x64 or x32), and the oldest only XP. As for why you'd want various Linux distros, I imagine it's a similar platform-sensitivity issue.
I wish there were a bundle pack of USB drives with installers for all Windows and Mac OS versions, so I could just pick out the right installer and install. And another bundle of live USBs. Carrying around lots of USB sticks would be bulky, but somehow I expect it to be more reliable than Ventoy - my experience with the Zalman ZM-VE350 has never been reliable enough to replace an external CD drive.