By who, your LFS that's looking to make some cash?
Protein skimmers filter out dissolved organic proteins or other hydrophobic organic compounds. Most of these dissolved organics are portions of fish waste and will release nitrates and/or phosphates into the water column. Protein skimmers remove these dissolved organics before they can break down and add nutrients into the water column.
Problem is, protein skimmers that are truly effective are large and clunky pieces of equipment that so not do well in nano setups. nano skimmers really dont do all that much and they can malfunction and release lots of air bubbles back into the tank which in turn can cause embolisms on invertebrates.
While I'm sure others disagree with me, IMO, a skimmer has no place in a 10g aquiarium.