The rigging portion of the character animation process is essential to ready the character to be animated. When rigged, a character will have defined joints, which will allow it to articulate in a realistic way.
A key difference between rigging for a game engine like Unity and rigging for a non real-time animation is that we do not need to set up full character controls in our 3D software.
On export, controls and other helpers will typically be deleted leaving just the mesh, bones, and animation (if there is any). This makes rigging a model for Unity a little easier.