Use case #1 You built a REST provider in outsystems and now you want to share it via Outsystems Forge as a component.
Each outsystems customer will install it on their outsystems platform and they will consume a different BASEURL for their server domain.
Use case #2 If you want to test your REST consumer eSpace, you might want to change the BASEURL to test it in sandbox or in live mode. Most commerial REST providers offer a sandbox and a live enviroment - the REST API structure defination is the same but the server and request URL is different.
Use case #3 You are an enterprise, you install product X on many enterprise servers, and now you want to build a component to consume product X REST API.
It doesnt make sense to clone and duplicate the component/functions hundreds or thousands of times to do the same thing! these problems will be solved with a variable text value BASEURL.