ISO images don't have to be bootable. In fact the original standard (Philips brown book?) didn't have it in the spec. It was only after El Torito that cd-roms became bootable.
ISO formats for cd-rom, dvd and that later abomination I shall not name can be useful in that you can shoot them and blank media straight into your burner and have something that can be readonly and used by some types of consumer media devices that are too stupid to be full blown computers (anybody remember CD+g and CDI?)
When thumb drives started to hit the market, they also allowed iso images to get blasted right onto them. When they were big enough, you could have the front end of the thumb drive be a bootable livecd distribution of your favorite Linux (Gentoo for me) and enough space for another partition that you can have as read-write (btrfs instead of vFAT for me) to go in and raid anyone's system no matter how big a mess winders and/or malware may have made of it. In fact, this is how people like me can unfuck the wrecks of dying hardware and/or bad life choices for others.
But you are correct about it being about as useful as an ashtray on a motorcyle for VN downloads. This is probably some site that's trying to make a quick buck for selling ISO software to winders folk since it doesn't come with out of the box. Us Linux types have it baked in to the software (which is also free to start with).
I can't remember which one it was, but it was the ISO download. Why would anyone do an ISO file? ISO's make bootable disk images.. big flashing NO to me.