For me I only used the service builder to create demo data and to do not forget important field. In the same way our customers are using it. Your second question I will split up, because I am unsure about your final target.
1. Why can I not create a business application or an application service?
At first I would like to take the definition from ServiceNow:
"Service Builder enables you to create and maintain definitions of your business and technical services and offerings through a guided experience. You can edit service definitions at any time before you publish as well as update already published services using version control."
So the purpose is the management of the technical service and business service in a guided experience.
2. Why can I not add the relationship between Service offering and application service/business application?
Based on CSDM the business application has no direct link to the service/service offering, so therefor we do not have a necessity.
For the application service it would be lovely to have the function to add an application service. Why this is not introduced is an unknown for me. But most often I would say, because you link the application service not directly in the creation phase, but more during the development phase. So you would most often create the application service after the creation of the business or technical service. And therefor we have a function within the maintenance of the application service.
I hope my answer helps you somehow. Please feel free to ask, if something is unclear.
Thank you.
