I rely on actions extensively to build functionality in my sites. I find them convenient for business logic because they are packaged chunks of functionality that can be called from many places, including Trigger, Rules, and VBO. Today, I've added a new place where actions can be called: the URL. Actions REST provides this functionality.
Consider a job site where a supplier receives an invitation to participate in a project. The supplier logs on the site and views the invitation, then wants to accept or decline it.