why does making a bootable USB have to erase all else first?  

It depends on how you create bootable USB?

If you create it with some program it will erase all data.
If you create it manually (ex. it's bootable from very start) and later you add some files, and then Windows installation files it will boot just fine.

I think problem is when there is a conflict as to what should boot from USB, in which case likely nothing will boot.
Solution is to keep additional files in folders, and bootable files on USB root.

Hello @Bob1212,

Bob1212 said:

I'll understand it more if I know the why of that. Why can't the necessary files just be added onto the USB stick?

Also, after making a bootable USB, there's lots of extra room left over. So can I write other files on that USB, say another copy of an ISO just for the sake of having another copy for surety. Or does adding more files somehow ruin the bootability?

One way to achieve this is to use => Ventoy

You can have multiple ISO's and Folders on a Bootable USB. This thread might be useful . . .

> Ventoy iso bootloader EASY installs from usb for any iso os file

I hope this helps.

Hello @Bob1212,

Bob1212 said:

Aha, that's interesting. I've been thinking I need to be able to run a very small Linux from a USB in case Win10 goes bad and I need to use Firefox and Thunderbird. Nothing else, in the short term. Would you happen to know if the very small Linux named Porteus (700Mb) would suffice? I did see you mentioning Mint and Ubuntu in that thread you cited, but I'd prefer something super small. Anything else you're aware of?

Unfortunately NOT.

You should be able to run Linux Porteus in the same way as Linux Mint and Ubuntu from a Bootable USB.

There are members who use Distros that might be able to comment.