Sometimes, modules or custom code needs to interact with a instance, either to e.g. create new issues under certain circumstances or to probably grab some status information or issue lists. Writing the code to talk to GitLab's API is cumbersome and a waste of time to do that over and over again.
This module is just an API module. It doesn't do anything on its own but it can be leveraged by other modules to easily get connected to any instance and to read or write data from or to that.