Spine is specifically for 2D. All the tools in Spine work for 2D and the data that is exported. After export, Spine has runtimes you can use to load and render the data in your games.

Maya (or Blender, etc) are mainly for 3D. They can be used for 2D, but have a lot of advanced features. Likely you need to write scripts to export the data in format you can load easily. You'll need to write runtime code in your game to load and render the data. If you use any of the 3D or advanced features (which is sometimes easy to do by accident) your script won't be able to export or the exported data won't render as you expect.

There are many tools to do a job similar to Spine (Maya, Blender, 3ds, even Flash). The biggest difference is the Spine editor has a workflow for building animations that is specifically for building 2D games and that Spine provides an end to end solution. You can go from building your animations to using them in your games without having to develop your own tools to export, load, and render. You can focus on building your games.

A friend, Roquen, once wrote:

If you can find a tool that costs "C" that does (or almost does) what you want...do the math:
1) T = Guess of time savings.
2) Account for opportunity cost: 2T
3) How much is my time worth: E
4) If 2TE >= C: buy and move on to important stuff.

When considering Spine, be sure to take into account that 1) it is still evolving and improving and 2) the licenses give you ALL future updates. It is our goal to provide so much value in Spine that developing your own tools would not be the smartest move, as that would take you a lot of time and have fewer features.

Just to clarify Unity and 3D assets: Unity imports the (or automatically converts) most popular 3D modeling and animation program files into FBX format which it can easily read, access things like animations, meshes, materials, weighted rigs, through code and UI, and optimize for its engine. Many 3D programs can also export to FBX.

But the fact remains, it's a different system for a different purpose.

Spine is for 2D skeletal animation, and both the Editor and the runtime code is designed to make it easy and to provide a lot of flexibility in a 2D-sprite context, both for the programmer and the animator. Most of the major benefits are outlined in their video.

Arguably, there are few (possibly none) commercially available systems that offer what Spine currently does. And there's definitely more to come; I mean, just look at the Kickstarter feature list.