The biggest reasons are being able to open projects and view all of the files in the project (and search through all of them) at a quick command. I really liked the themeing functions. Multiple OS compatibility (I regularly switch between macOS and Windows), so I needed a client that is used basically the same on both OS's. And one license is for all of the computers you use, and on all operating systems.
The more I use it the more I like it. Sometimes I have files that still open in N++ and I quickly switch out of it into sublime. Oh, did I mention at the command line (in OSX) you can open files using the subl command (Very awesome).