Steam is a third party site that allows you to play your games on just like Epic, Xbox, Ubisoft, EA, Origin, Battlenet, GoG. Steam has free games that indie developers have made along with triple A games. Amazon games uses steam also for games like New World and Lost Ark.
You can get skins from games that goes into your inventory on steam and you can either turn them into gems or sell them for a few cents. I think this perk is nice because you can basically get rid of things you no longer want and make a couple cents off of it which in the long run, can take a few cents to dollars off of a game that you are wanting.
Trading cards can be traded to friends or you can sell them. If you do not want to sell them you can crush them into gems which when you get 1000 gems you can sell them or if you have a game you are trying to make a booster pack for you can see how many gems it will take and if you have enough, you can get them. If you have duplicates you can crush them, sell them or wait. You can hang on to them until you get more cards and you can keep leveling up your badge for the game which is nice.
You can also buy your friends games and give them a digital gift card within the launcher. You can also get a refund on your games if you do not like it or if there is something wrong with it.