I use a Mac for software development because it's quite simply the best platform for coding and development.
It's a native UNIX machine, you can drop straight into bash and have the whole of UNIX at your fingertips. If you need extra packages you can install Homebrew and pull in any packages you want
Homebrew ? The missing package manager for OS X
I use Sublime Text 3 & Atom for text editors.
Git command line tools and the Git GUI
If you run VirtualBox and Vagrant then you don't need to leave your machine to do development, you can fire up whatever native platform you are developing web apps for right on your machine.
You can test all your code locally - regardless of where your end product will go - set up a dev server on Digital Ocean or Vultr etc and quick deploy git changes to that on your dev branch for immediate remote testing and auto deploy your master branch to a production server.
That's why I use a Mac, because it's quite simply the most efficient tool there is to get shit done. Development on a Windows machine, especially web development sucks.
Every solo developer I know uses a Mac.
Parallels is great when you want to fire up a desktop OS and either integrate it or isolate it from your Mac for work or just testing.
The beauty of VirtualBox & Vagrant is that you can have your entire codebase local and test it as if it were on a production server. So you can have a Centos 6 Instance using CPanel and your code base on your local machine and test that your App will run properly in that environment. Or run Ubuntu, Fedora or whatever other production environment while keeping your code local.