Command-Tab is great if you only ever have one window open in each of your applications. With more than one window, though, it's a hassle to find the one you want. Witch solves that problem by taking you directly to the window you want to reach.
If you keep a lot of windows open in a number of applications, you've experienced the frustration of switching between those windows: Command-Tab to switch apps, then cycle windows within the app to find the one you want. If you're lucky, you can cycle the windows with the keyboard, or you may have to go digging in its menus or Dock icon to find the other open windows.