My main reason is so I can surf the Internet while I watch Netflix on my ipad 3 at work, without using all of my iphone 5 battery. This is a huge plus for me! And the portability of course.
I had a kindle fire, but I lost it like a week before the iPad 3 came out. It was clearly gods way of telling me to buy an iPad 3. Anyways, I bought an iPad 3, but I still liked the size and one handed ness of the kindle, so I bought an iPad mini too.
Never owned an iPad or any tablet for that matter. Don't like other non-Apple tablets but always thought the iPad was too big. So the mini was a good deal for me.
Since I switched to an Air I never used my iPad, they are just too similar in size and convenience for my needs. I did still want a small device I could take to meetings to take notes and grab data on, so it fits well between my Air and my iPhone 5.