A skimmer is the only filtration method that removes dissolved organic compounds BEFORE they begin to be broken down by the biological filter which results in less work for the BB to perform and lower nitrate and phosphate levels overall.
If you can put a skimmer on your system, you should.
You can run a successful aquarium, fish only or reef, without one but the benefits of a protein skimmer are unique and not to found in other filtration methodologies.
All filtration methodologies deal with the breakdown products of fish waste, a skimmer removes it from the equation completely.
ps, they also help with oxygenation and liberating CO2
from the water.