The user profile is sort of the home base for each user. You can access this by clicking on the shield in the center of the gate with your House mascot.
Here you can see your wand, your progress through the story via your pet avatar, your favorites, your art, potions ingredients, etc. Pottermore allows users to upload original drawings and paintings, and will also assign some fan-made pictures to the categories seen in Pottermore, such as various spells, potions or creatures.
You can also access the textbooks and chocolate frog cards you’ve found around the story. It’s an easy way to tell if you’re missing any so you can go back and continue looking. Sometimes fellow Housemates will be willing to post hints in your common room as to where to find them if you’re really stuck.
You can also see your friends and accept more friends from this page.
Overall, it’s a great way to keep everything that’s tied to your account together in one easily-accessible place.