These little 3-minute specials are a direct sequel to Amagami SS+ plus, but you can interpret it as "afterstories" to the arcs of the parent story. I enjoyed all the specials despite it being shorts because I'm a fan of the show. So that means that if you really loved any of the heroines, and want to see what happens to them further on in the story, then these lovely shorts should satisfy you. These specials will give you a conclusion that goes beyond the conclusion given in the parent story.
OR... another reason for you to watch these could be to indulge yourself in nostalgia towards the heroines reappearances again.