Once you have a full set of cards for a particular game you can craft a badge. A badge is really nothing special, it's just something on your Steam profile that indicates you liked that game enough to collect all the cards for it. You do get XP for crafting badges, which increases your Steam level, which allows you to add more friends.
FWIW, I would sell the duplicate cards instead of making gems. I have done both and the investment in gems is much more of a hurdle than making $.10 per card. Just make sure you look at the graph to see what they are selling for. Your profit has to be slightly below that (so they pay $.10 but you only get $.08) but the cards will sell almost immediately.
Edit: you need booster packs because you cannot earn a full set of cards from playing a game. You have to either buy the missing ones or get them in booster packs. However, booster packs will usually contain duplicates of ones you already have so buying them on the market tends to fill in the holes much more efficiently than hoping for booster packs.
Last edited by DWMarch; 12-27-2014 at 03:00 AM.