MuleSoft why it has API designer when this is integration platform

I am exploring the usage of MuleSoft and I am new in using it. The MuleSoft is integration platform, then I am wondering why it has the functions of API Designer (API create service)? Because having an API Designer in an integration platform may lead to create business rules as implemented in the API created in MuleSoft! The integration platform would not need to have a facility to create API, rather just a way to call API and manage API request which is provided by API Manager. The API creation shall be done in the other systems which MuleSoft will integrate. This could make integration platform doing more than what it is intended. Or I am missing something? Wanted to understand this aspect of MuleSoft.

The reason could be to give some quick way to use functions of MuleSoft, by directly having an option to create API in MuleSoft itself. May be?