Most sparky user don't need to justify their use of Sparky. We know what is good and what is bad in the Linux world followed by many years of Linux use. But, if I had to narrow it down to something, I would have to say it's "choice." Choice to decide what you want and don't want in a distribution, something that lacking in a raw Debian edition. Therefore, Sparky gives you this choice, it has great security, privacy, has a large section of apps and allows you to customize your operating system/desktop as you please.
Lastly, the Sparky forum is very friendly to both old and new users. If you post a problem, a suggested solution is using posted quickly by another user. What else can you ask from a distribution? Oh, it's free too.