In many cases, to optimize or improve the system solution, the business decides to integrate with external systems. External systems have their life cycle, state, and communication protocol. There are different types of communication protocols, in this article, we will only consider HTTP. Microservices also use synchronous HTTP calls for notification or to achieve strong consistency.
You definitely want your integration to have fewer problems and be able to diagnose them easily. In order to implement reliable communication between the internal solution and the external system, it is necessary to cover the communication protocol with reliable integration tests.