Similar to most "Half-stories" or "novellas" or "short stories" that authors write to fit in between the actual series novels that they have written, this is nothing necessary. 
sure it gives a bit of a better background to a character and maybe a handful of readers will enjoy knowing a bit more about him. 
But is it necessary for the entire series to read this?
No. 
Was it necessary or important to write this and it added something to the story that made it so much better?
Absolutely not. 

I actually -most of the time, lets say that- dislike most of those little add ones to the series that Authors and/or publishers are now to fond of. 
They very rarely add actual necessary details or background to the story itself and are just something that people that feel the need or want to completely finish a series slog through. 

Was this horrible?
No. 
Even if i might have just made it sound like that. 
But it was very much not needed and if you haven't read this, don't bother.