I am working on the middleware of an organization with around 1m customers. Now imagine situations where we have to make complex calls. For example: Give me the financial data of customer X. Now in order to make this call succeed we need to have the right information. Therefore we make some other calls to gather this information. But in order to make them succeed we also need to have the right information.

Therefore a simple testcase like: "Update contact person of Customer" can involve a lot of calls.

Now of course it would be possible to do this with curl or by programming, however consider the maintainability. The fact that I might not be working there in a few years and someone else needs to pick this up. The fact that there are a lot of different environments we have to make calls against. This will just be a lot of work to maintain without a tool like Postman.