I do not have one yet, but I am going to get one. The reason being, after watching a video from Springfield Leather. They appear to have the ability to cut fairly tight curves, along with long flowing curves with ease. I don't think you'd do it quite as easy with a razor knife.
The second big reason is skiving, and that is a huge one for me. The video shows Denny skiving a piece of leather and does it while making it look easy!
The video is a bit long, but I think it points out the questions asked quite well.
https://youtu.be/8ARqjhTGYR8
Terry