Rails - `content_for`

The purpose of combination of using content_for and yield is to allow you to inject data into the application layout form ANY view.

For example, in your application.html.erb file:

<% if content_for?(:navbar) %>
  <%= yield(:navbar) %>
<% else %>
  <%# default navbar %>
  <section class="navbar"></section>
<% end %>

And you can change the navbar from any view by using this:

<% content_for(:navbar) do %>
  <section class="navbar"></section>
<% end %>

==> You can custom view for each page with no pain.