There are special missions that become available as the Reaper War grinds toward its climax. They do not advance the plot the way the Priority missions do, but these operations are still important to the war effort.
War Assets are often secured or improved through these missions, and side plots, sometimes involving characters returning from previous games in the series, help flesh out the story.
The catch here is that these missions are very time sensitive. Once you receive the assignments, you have limited time in-game to complete them.
If you deploy on too many missions, these side jobs will no longer be available, costing you valuable resources.