What is the difference between drilling wells automatically with Automatic Development and Well Cloning jobs?

Automatic Development has a reservoir driven logic. The wells to clone are those of a reservoir (or a subset of them). Well & Facility Cloning jobs do not refer to any reservoir - it clones a pattern of wells which can belong to different reservoirs.

Why do we need both?

Automatic Development is a convenient way of driving well count automatically depending on reservoir reserves. However, you may want to develop a pattern regardless of any limit imposed by a particular reservoir either because you are developing with it just a part of a reservoir, or because each pattern is associated to more than one reservoir. In such cases Well & Facility Cloning is a less specific functionality, more flexible, not tied to reservoirs, though lacking all the control features of Automatic Development.

See further under The Concept of Cloning.