Depends on the size of the game and your internet speed. If both kids want to play Ultima 1 and you have a 1Gbps internet at home, you might choose to download the installer twice on both PCs, instead of copying the file around with an USB memory stick, especially if you don't have one available.

I think it is irrelevant for that case, the point of the issue was that the required client (in this case Steam) wouldn't allow both kids to play the game at the same time, at least without some extra hoops. How the game got installed on their PCs is less important for that issue.

Not at all. For instance, you bought The Witcher 3 GOTY at your home over your data capped and slow GPRS 2G phone connection, and it has been sitting there in your account for a month when you finally get an itch to install and play it. Then you go to the local library or your friend's house, download the installer files for that one game to your USB memory stick, and bring it home in order to install it.

Not sure what exactly you meant by "backing up". If you meant routinely downloading all your installers even when you are not going to play them in the near future (which is also what I think of as backing up), then I don't see that is required for either of those cases.

If "backing up" just means the process of downloading even a single game on some other PC so that you can then transfer it to the target machine and use there... ok, but I don't consider that as "backing up your GOG game installers". You might just as well delete the installer right after you have installed it, at which point it is not backed up anymore.

"Around here"? I think most GOG users don't back up all, or even most, of their GOG game installers. Most most probably just download game just before they want to install it, and will maybe care about backing up only if it seemed GOG might be closing its doors, the same way DotEmu and Desure did.

I didn't have a Desura account but I do recall many people here rushing to download their Desura games when it hit the news it is closing up, and the same with DotEmu.

I also most probably wouldn't have all my GOG games downloaded, if it weren't for gogrepo (or lgogdownloader). I tried it once manually back when I had like 500 games or so, but I found it a too laborious task, especially trying to track which games had received updates that I should also download. With DotEmu it was much easier as I had only maybe two dozen or so games there, and they very rarely updated them (I recall seeing some of my DotEmu installer files changing only once).