Your default price catalog is the catalog you use e.g. for the products you sell on your own website so it is important that it contains retail prices. Other wise you'll be out of pocket.
If you have for some reason made a contract with an agent that contains net prices, or if you offer any of your sellers prices that are different to your retail prices, you can create a separate price catalog for that agent.
Generally, there is no need to create a second price catalog. In most cases you can assign your retail prices to your sellers, they then receive their commission of the retail prices.