Drip edge is metal that helps prevent water from running behind the gutter, which can rot out the fascia boards. Drip edge metal is bent to the correct angle. Then it slides under the shingles but over the back of the gutter, so that water runs from the shingles onto the drip edge and from the drip edge into the gutter.
It cannot be predicted in advance whether drip edge will be needed or not. Drip edge may be needed for several reasons, but the 2 most common ones are that shingles may be cracked or broken, or that the gutter is sloping and is very long. At the end of the slope there will be a gap between the bottom edge of the shingle and the top of the back edge of the gutter, which allows water to fall behind the gutter.