The first reason for building a marketplace is the diversity of revenue strategies. There are many monetization models to choose from: commission, membership, listing fees, advertisements, etc.
Second, online marketplaces are known for their quick expansion. Provided that everything is working fine from the beginning, the number of buyers and sellers will increase rapidly.
Finally, since marketplace owners do not have inventory, they are free from any issues that come with its management. The same goes for delivery and logistics. Vendors are dealing with these processes. The marketplace owner is responsible for providing an online platform that connects buyers with sellers.