I don't get this. How come there are keys for free games and is there any difference from actually getting the free game directly?
For one, some of these games, not only were they commercial - hence you had to pay money for - but they also had cards to them, which wouldn't come if you just added the game to your library like that. Postal 1 is one example of this: It initally cost money, but turned F2P near the end of 2019, so, if you want the cards it drops, you either have to buy the Steam version (or, better yet, buy Postal Redux, but your choice.) from somewhere else and activate its key or get free a steam keys of the game, like in this thread. Another difference is that if you just add the games to your library, they won't increase the number of the games you own in your profile (aka the F2P versions of them, or games that have been F2P since Day 1 aren't +1-s).