You can sell copies of your books there and once you've sold $50 they'll cut you a check or pay your PayPal account. Go to your Author Dashboard and find the book you want to sell. Click your book, then when the page loads you can see in the upper right your "Book Stats" and "Upload Ebook" which is the one you want. Make sure you look on the upper right to post your book cover image when the page loads
If you just want to add some free writing, on your Home Page, go down to your profile section and "writing" is listed there. Above you profile is where you can add status updates, btw.
You can set up giveaways and Q&As and all kinds of advertising stuff. You can even advertise with Goodreads, though that costs money. Make sure you link your blog posts too because as a fan of like CJ Cherryh, I love reading about her real life.
Just connecting with your possible readership is invaluable.