achievementssteamsteam-trading-cards
From the faq I know that the following give XP:
What other activities give XP? Are there any ways to get XP other than getting badges? Do regular achievements for Steam games give XP?
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.
Crafting a badge from a full set of cards gives
So, yes, there is quite some use to the cards. Whether or not it is worth it is an entirely different discussion.
Best Answer
From what I've seen so far:
Steam Special Events appear to grant XP, although the formula is unclear to me. The individual data is incomplete for most of the previous events. Ones that I can confirm count for some amount of XP are:
For community contributions there are other bonuses (with thanks to Koraktor in the comments):