Having two drills means you can leave a different bit in each one and speed up the work!
Many woodworking projects require you to countersink screws, or at least predrill a hole first so the wood doesn't split. With one drill, you're constantly switching between a drill bit (to make the hole) and a driver bit (to drive in the screw). Having two drills means you can leave a different bit in each one and speed up the work!