One word: Demos.

lol funny. But I haven't touched it though. Space Rangers, Druidstone and one of the Battle Chess are the only games I played.

The Elder Scrolls: Arena and The Elder Scrolls: Chapter II - Daggerfall. I didn't want them and I probably won't ever play them.

Other than that, just a handful of duplicate items. I have yet to dislike a game enough to hide it from view. I haven't touched demos so far, and I pass on freebies if I'm not at least somewhat interested.

I do that as well. I used to hide games I didn't like too, but I eventually just decided to tag them appropriately. I might still decide to go back to hide bad games, since we don't have a way to exclude certain tags from showing.

Mostly games I got for free and didn't enjoy and some games I got for my two nieces and nephew when they lived with me (they moved abroad a couple of years ago). I've avoided demos because I can't delete them.

Delores: A Thimbleweed Park Mini-Adventure is hidden because it won't work on my PC, so it will eventually go back to the main collection.

Deponia is hidden because after I purchased the game on GOG, I got the series for free on Steam. :/

Wasteland 2 and Planescape Torment are hidden because the Director's Cut and the Enhanced Edition are on the main collection.

I use the Hide function mainly for filtering out demos and freebies (of the sort I'm unlikely to play).

I keep about 50 or so games visible at a time, with everything else hidden and categorized. Some are backlog games, and many are games I've finished with no replay value. Others are just not fun, and the rest are demos.

Demos, artbooks, goodies, prologues, and Bad games (Like Lumo and Moonlighter)
Only 6 in total though.

So far none.

That's true of course. My thought process was that, if content is made available by way of streaming not all files will necessarily be played at full length and only those songs, videos and wallpapers someone is interested in would be downloaded instead of one large junk of data and possibly higher traffic.

Being able to access a gallery, read .pdf online or within the client environment, listening to and watching movies and downloading individual items, should lower the overall traffic on GOG's end.

Soundtracks are offered as mp3, wav or flac formats, sometimes in all of these formats. I should think that for streaming they are going to use mp3 as standard (lower file sizes), while at the same time offering all available formats for download. This should then benefit customers with low connection speeds. It would take ages for them to download a packed 2.01GB file, which would be the Desperados III soundtrack as a case in point, than being able to access and listen to or watch even on a smaho if they so wish to.

I've only hidden that Jagged Alliance freebie from years ago. IIRC it was very early on and GOG just added it to everyone's accounts without asking.

I only hide the demos.

I also use the hide function only to hide demos.

I only have 4 so far.
The 1st is the original release Regalia: Of Men and Monarchs. There were 2 DLCs that wasn't sold separately on GOG for some reason. Instead, GOG has Royal Edition, which includes the DLC. If I remember correctly, there was discount for those who already owned the original.

The other 3 games are demos.

Crossing Souls - got the full game
Star Vikings - didn't buy the full game yet
Hellish Quart - waiting for the game to leave the development stage