Explained this IRL already but just for future people who may ask:

This essentially escapes ERB. This file is being used as an ERB template to generate a new site's `index.html.erb`. Since we want `<%= render "partial" %>` to appear in the generated version we have to escape it with an extra `%` so it doesn't try to run `render "partial"` and actually embed the markup from the partial in the generated version (or, more likely, error because I didn't include the right file to make `render` work or something).