.spn files can be previewed in the browser and are also offered for edit in Planner (Live) this option allows you to edit the planner file content directly in the browser.  It opens it in the Planner live part of the application which also covers it's check in.  see here for further information on this.

Higher level functions of the Planner file are only available in the Planner app for desktop but basic edit and addition of tasks, resources, risks and issues can all be done in Planner live.