To really answer the question, you have to understand what user problem is it solving.

Lets look at it from couple of different angles. First a solo developer. If you've ever developed against an API, you know how difficult it is to pass headers, cookies, change GET/POST/PUT requests. You might want a way to repeat a previous API request again and again. You might want to create a collection of requests that you can refer again. If you're using curl, you'll soon start having a myraid of shell scripts with variables and control flow. At some point you might want a UI, and soon you've invented Postman/Insomia and the likes.

Now imagine you're a team. In a team, there are cases where you might want to share "requests collections" with others in the team. What are the options for headers, what flag configuration works best for a certain scenario and the like.

So yes, while postman looks like curl, it adds so much value over curl and that's why it exists.

It will be similar to dismissing dropbox/google drive, because rsync exists (yes, I'm aware of the HN lore)

Postman is a graphical tool to create, design, and test web-based APIs. cURL is a communication library that has no graphical component.