When you want to install a new OS, you usually create a bootable USB flash drive with the .iso of that OS. To do that you can use GUI programs like Etcher or Rufus or the dd command. Or maybe you’ll take the more convinient route and create a multiple-ISO bootable drive.
But, maybe, you just want to create an Ubuntu LTS bootable flash drive and just throw it in your backpack or hang it on your keychain, just in case you’ll ever need it. Maybe, for any reason, you don’t want to download the iso or image to your HDD/SSD. Maybe you’re out of space. But you really need to make a bootable drive.
You can just write the ISO files directly to your USB drive as you download it from the internet.