Why download to an ISO rather than to a flash drive?
IN addition to what's been mentioned above.....
It's become more or less a habit to me, to have the ISO on hand.
Back in the day, before 64 bit became so ubiquitous, I would often generate both 32 and 64 bit installation media separately. Now you can host them both in the same ISO and on the same installation media, but....
I still find myself generating installation media for older MBR / Legacy BIOS systems separate from GPT / UEFI systems.
It just seems more convenient to keep the ISO on hand.
With kids, grandkids and friends like mine you never know when one of my Thumb Drives will sprout legs.
Being able generate installation media from the ISO in a few minutes onto a new medium is just handy.