Basically, collecting each card for a game will allow you to craft a badge, earning you experience which can increase your Steam level. Your steam level allows you to customize your profile, adding backgrounds, adding "showcases" which make it easier to show off what things you've accomplished (like achievements).

You level up by making badges To made one badge you have to use steam cards Cards drop from most of the games (Not all) on steam. They drop when you play a game You only get half cards requied to make a badge rest you have to buy on market or trade You can also get steam cards from Lvl bots they give you for example cards for 20 badges

It’s the place to buy and launch games and join up with others using the same system. Each Steam account has a “Level” section. The leveling system intertwines the platform with the achievements you earn playing. Once you reach level 10, you can customize a showcase for your profile page. Then, every ten levels, you gain additional showcases.