Generally speaking, baking involves any process that allows you to trade in Memory for CPU.
In other words, baking "cooks" a procedural process and caches it to memory. One example is AO baking. You can UV map an object (another use for which UV mapping is indispensable
) and then bake the AO pass to a texture. This is generally used for game engine applications, but can have more general texturing uses as well. I would rather bake AO and use it as part of a UV mapped texture (in some cases) than have to render that pass ever time.
Normal baking and texture baking have their own applications, providing data you might not otherwise have access to, such as with procedural textures. Baking animations can involve putting keys on every frame, where otherwise procedural interpolation would take place between sparse key frames.
Ultimately baking Orco mapping may seem superfluous, but it would be nice to have the option, as sometimes you have exactly the mapping you want with Orco (such as with a good spherical mapping) but then might want to switch to UV mapping. It seems a shame to start from scratch in this case.