Lenses with a built-in focus motor can autofocus even if the camera does not have its own focus motor.
Lenses with built-in focus motor focus faster and more quietly than lenses without a focus motor which rely on the camera's body focus motor.
With full-time manual focus, you can move the focus ring whilst it is in AF (autofocus) mode. This means that you can make manual adjustments once the AF has finished, without changing to manual mode.
This is the closest distance that the lens can focus. A shorter minimum focus distance allows you to get closer to your subject, and is particularly important when doing macro photography.