It's very handy when changing lenses in a haste, or in subbed light; you can feel by that white little half ball where and how to alight the lens while keeping an eye on the subject's situation.
The body's bayonet's starting point is somewhere in the middle between the mirror up lever and the underside of the camera, where the lens's unlock button is, and is marked with a red dot on that bayonet (which isn't so easy to see).
On the Leica lenses it's red little half ball...