Curl is great for running individual API calls, API clients are great for when you're actually working on the API or architecting an app with one. Not that there are things one can't do with API development from the CLI, but there is a lot more to API clients than that feature list (it doesn't even include anything around stuff one might want to do with openapi definitions!) and by the time you string it all together you get why people like the tool that did that for them instead.