It's Used for Previews
At the time of this writing, Octopress has a :preview rake task that invokes rackup, which in turn invokes Sinatra. Sinatra is used to serve up the pages on your localhost for viewing.
Question
I'm currently getting started with Octopress and just cloned the repo. I noticed in the Gemfile that it requires the Sinatra gem.
Why does Octopress need Sinatra? I assume it's only for development.
Solution
At the time of this writing, Octopress has a :preview rake task that invokes rackup, which in turn invokes Sinatra. Sinatra is used to serve up the pages on your localhost for viewing.