Are they needed, no but they are definately recommended. Especially if your just starting out. Skimmers remove a fair amount of DOC
which is where the nitrogen cycle starts. The less DOC
in the water column the less NH3
the biological system has to deal with.
If employing alternate methods of export such as weekly water changes, planted refugium for harvesting macro alges or a regularly changed resin/carbon routine; a skimmer becomes less necessary. They do however greatly reduce your overall maintenace leaving more time to actually enjoy the tank