Buyers enter the requirements of how they need to use content in the project regardless of any one particular license. Sellers enters the terms of how they are allowing others to use their content in their license.
For example, one photographer might indicate that content is available for licensing for a certain fee regardless of any other details of how it might be used while another photographer might restrict that their work only be used in print and in the US.
A single buyer may need to license content from both of these photographers, and possibly many other sources, so would want to enter all their required terms and details in their project. If the buyer were creating a website and entered that in their project details they would be alerted when they added content from the second photographer who only allows print projects.
To further complicate matters a single piece of content may have multiple licenses associated with it, say a photographer and a model, which might each have different terms and requirements.
A buyer that just wants to buy a single royalty free image under a project and be done doesn’t have to fill in the terms and details of that project and will be in effect agreeing to that single image’s license terms, but some sellers may request more information to ensure the buyer understands the terms they’re agreeing to.