FTFY. Also, trading card drops for queue exploration is pure evil.
Fortunately, I don't have any remaining funds in my Steam wallet, and I've managed to be too lazy to add my new credit card to Steam for quite some time. I estimate a 50% chance that I'll make it through the sale without any purchases as a result. That said, Grim Fandango Remastered is very tempting, since I've never played the original one. As for fondly remembered games, I'll probably be in trouble if Fallout Classics Collection gets more than 40% off...
"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram
What's the point of the trading cards? I've always just turned them into gems since they expire, but I don't really know the point of that, either.
The summer/winter/etc sale trading cards are similar to regular trading cards with a few exceptions, mainly the expiration date you mentioned, and that there isn't (usually) a level cap on the badges you can craft with them.
(If you were asking about trading cards in general, the badges you craft will let you gain levels. Each level increases your friend cap by 5 (up to 2000), and every 10th level gives you a nice showcase for your profile page.)
Gems can be sold on the community market, or be used to create booster packs, each of which gives you three trading cards for a specific game.
"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram
I don't think I've even messed with my profile page. I didn't even have a full profile until I created a second account for loaning Space Engineers to friends.
I'm happy I'm currently not playing any games that I load through steam because I'm pretty sure if I did I would spend way more money then I really need to right now. I got overwatch recently and that was enough money spent on games for a year.
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"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram