I was reflecting a little on the authors' previous work, Seikon no Qwaser, and even if i like that series very much i have to admit that the last arc of the story was really underwhelming. But looking closely to the series as a whole i noticed that the problem started in reality halfway through the series, it felt like the authors were losing their passion for that manga, and even with all their effort to end it somewhat well there is so many plot thread and details that weren't well handled or addressed anymore after some point.
Were i am trying to go with all of this, is that i think the author learned from that and are trying to avoid this happening again and these extra chapters are one of the countermeasures for that, they give the story( and them ) some space to breath to address other characters in a good and satisfactory manner, they want us to love their character but learned that they need to give them a "character" to be loved, and even with so many loved characters in seikon there was some point that loving them felt like obligation.
I am really happy that they are putting this effort for their readers and for themselves because in all this i felt too that the authors were losing their love for their story and this i think is a really scary thought for someone that creates something out of love.