Boredom and because one can always lose access to the internet at any time for various reasons? ;)

Plus I usually download games when I buy them but I forgot to with a few of them(same with other game sites) and since I didn't know which I had backed up recently due to not keeping track of when I downloaded what, I decided to download it all over again, for those reasons & to also make sure I had up-to-date versions of everything.

Keeping a local backup of content you've paid good money for, or is otherwise valuable to you (like personal photos you've taken and want to keep), is only sensible. That is news to some people?

EDIT: Of course with digital content that depends on internet (like rented DRM games) it is of much less concern.

THIS....and with HDDs(usually) being cheap for massive sizes it's not too hard for many to save up and get a large drive for storage/backup along with your regular install/games drive.

Irrelevant. If I made it myself, of course I back it up. But if I didn't, I see no point to it.
I paid good money for the licence to use the ones and zeroes, not the actual ones and zeroes. Huge difference there.

I assume the people who used e.g. Megaupload to keep stuff they value "in the cloud" learned that lesson, unless they were sensible enough to also keep local backups too.
Pure semantics. People value some digital content, no matter where it originated. Either they made it themselves, or paid good money for it, or several other reasons. It has nothing to do with "backing up the internet", it is "backing up content that you value".

Yes you should back up all your GOG games immediately after purchase or at least soon after, so that if a zombie apocalypse happens tomorrow you can concentrate on two things only: killing zombies and playing your GOG games in between, instead of on two more things: worrying about internet connection and losing a chance to back them up while fending the zombies :-D