I got eight cards in a row on the first day I played after the update came out (including one duplicate). You automatically get about half a set of the lowest level for a particular game for nothing but playing it for a while (Random Drops).
Then Steam will show you that you could “easily complete the set to reach the first level”, simply by “trading cards on the market”. “Reaching a level” means that you get a wallpaper image for your Steam user profile and NMS emoticons for steam chat. Nothing of IRL worth, just a means to display your fandom.
There are maaany rules how you can produce cards out of thin air, or get rid of unwanted ones, or turn them into another currency with which you can gamble again to get more random ones… If you are a completionist it might entice you to spend real money from your steam wallet to buy missing items off someone else…
My experience is based on my completion of the first level for Space Engineers. I never used real money from my Steam wallet, I merely sold duplicate drops and earned, literally, pennies, with which I bought the missing cards which also only cost pennies. (SE is an exception, it is suffering from massive inflation because it continuously drops Steam-tradable objects as exploration rewards in game while you play, in little boxes with parachutes, it’s very cute, space helmets and tools etc.)
So, for SE it was easy to complete one level of card trading without spending any money, with No Man’s Sky, I don’t know whether I want to do that again. Here is what you can gain for completing levels: