What do Steam Profile levels do?

The higher your level the more "showcases" you can add to your profile which includes:

You'll want to join the Steam trading card group to level up your profile.

Joining the Steam trading card group is no longer required.

On May 15th, 2013 the Steam Trading Card system went into Beta. In order to gain access to the beta, users must join the Steam Trading Cards Group on Steam and will then receive an invite based on the time they joined. Being a part of the beta will change the way your profile looks to the new updated style.

From the Trading Cards FAQ:

What is my Steam Level? Your Steam Level is a summary of your badges and shows off your trading card collection and participation in Steam events. Each badge earned increases your XP, and every 100 XP gives you an added Steam Level (at least to start).

How do I add a showcase to my profile? Once you reach Steam Level 10 you can pick and customize a showcase from the profile edit page. Each 10 Steam Levels earns you an extra showcase, i.e. two showcases at Level 20, three showcases at Level 30, etc.

What are the direct benefits of my Steam Level? You get more options to customize your profile, and the maximum number of friends you can have on your friends list increases.

Higher Steam levels help with:

• better drop rates for booster packs - Every 10 levels gets you a 20% increase in drop rate. At level 50 you are getting 100% increase in the drop rate for booster packs
• profile customization - everybody gets the same set of choices it seems but you can add more showcases as your level increases. (I had 3 showcases at level 33. Another showcase opened up at level 40, 50, and 60 so I'm confident you get 1 additional showcase for every 10th Steam level.)

Levels provide the following benefits - every level you get will increase the size of your Steam Friend's list and every 10 levels you will get access to a Showcase on your profile.

More details about the stuff you can get on your profile can be found here http://steamcommunity.com/sharedfiles/filedetails/?id=145245037