you don't HAVE to use a skimmer, but some people prefer to as it can help with removal of DOC (dissolved organic compunds) which will help prevent unwanted algae growth, as well as oxygenate the water, add some (not much) volume to the system, as well as some (not much) current if it is a hang on the display type.
I personally use one, and always have, but there are others that dont. A friend doesnt on his tank, and my water quality is far superior to his however this is not always the case. Some skimmerless tanks do VERY well. as skimmers can also pull out some of the good along with the bad. Some run a skimmer, but only at night, or for a couple hours a day. Its really up to you.