Skimmers do not come with a built in pump as far as I know. Whether you use an external or submersible, doesn't matter much but something has to pull the water in. From what I have noticed, people with bottom drains often have a larger pond, let's just say over 2000 gallons. When the pond is real big, they will have more than one bottom drain even. So if they have a skimmer and bottom drain both, they have to put a Y in there somewhere to pull from both places. I didn't want to have a vortex and external setup so went with a submersible and no bottom drain. My pond is in the front yard and there would be no place to hide the out of pond stuff.