“ At least for me it is convenience, why I don't download them right away, if I've e.g. bought a dozen new games on a sale. I might be using my work laptop for the purchase and don't feel like downloading all the installers right away to the laptop, only to somehow transfer them to my main PC or the USB HDD where I keep my installers...
Also, since the games get updates all the time, you'd need to redownload most of them later anyway, several times even. So why the hurry, if you are not going to play them all right away anyway?” - then use steam. No need to bother with downloading or updating at all until you need to play them, plus you get all the benefits of bigger library, cheaper prices etc. As for updates I have a lot here, and I just once every few months and download what I need, half the updates aren’t updates, just gog galaxifiying installers, and a fair few don’t need to be downloaded anyways.
“ The version in the 2015 message is not actively maintained anymore, I think? One should seek the kalanyr branch of that, and IMHO use the development version of that as it has more useful features.” - yeah, I don’t use it myself, like with most open source you will spend most of your time finding out which branch or fork you need and which set of distributables or software you need to get it going. Proactively downloading as you buy is the simplest way of proceeding.