Pond skimmers serve two very important functions. First, pond skimmers house your pump which in turn greatly protects your pond pump by way of mechanical filtration from debris clogging its very important impeller.  Second, pond skimmers greatly reduce the amount of floating debris that fall into your pond.  Debris that will ultimately settle to the bottom of your pond causing timely maintenance issues. 

In addition to these functions, a pond skimmer neatly tucks away all the plumbing and hardware necessary for the circulation of your pond.  So rather than having the eye sore of a submersible pump and tubing traversing through your pond, you get to enjoy all the beauty of it.