To start, you first need to make sure you have loaded the Subtext app from the Edmodo store. It's free and you share it with the specific class you want. Now you're ready to go.
You will see this when you log into your account by a letter "e" by the group's name (If they don't appear, you may have initially logged in with your Google Account. You can either log-in using Edmodo or, while in Subtext, click "Settings" in the bottom left corner. Then, choose to connect with Edmodo.)
Now here come the 3 Reasons
If your students have paper copies of books, you can purchase one copy to put on Subtext. When you tag your copy and share it on Edmodo, the book notations appear in the Edmodo stream for students to respond and discuss.
When Edmodo is used, students can locate the exact comment from their peers and respond directly to it. This keeps the conversation continuing in a logical sequence (Currently within Subtext, whenever a student comments, it goes to the bottom of the existing discussion stream. This doesn't let students respond to a specific comment that may appear earlier in the conversation.)
After a particular article or chapter, I've posted specific Subtext comments to Edmodo and then placed a writing assignment to turn in. I usually create the assignment first and then post from Edmodo so the assignment appears underneath the post.