I have two reasons other's haven't mentioned yet.

1. Ease of site selection. Rocks don't matter. Level ground doesn't matter. That convenience is huge. When I don't have my hammock I'm always surprised at how hard it is to find a decent place to sleep on the ground.

2. It's drier. You already know that a tarp is drier than a tent because there is less condensation. Now picture being able to be up off the wet ground while still under a tarp. I've slept in torrential rain storms that left the ground a swamp. I was high and dry in my hammock.