What does a skimmer do, why do I need one?

What does a skimmer do, why do I need one? Can it go inside my tank as I don’t have a back compartment?
 
They remove organic waste from the water before it has a chance to break down and affect the quality of water. They also help reduce nitrate levels, limit excessive growth of algae and improve water clarity.  Proper maintenance needs to be ensured as these are sensitive pieces of equipment.  Maintenance is easy and just requires regular (weekly) cleaning of the collection and skim chambers.