There isn’t much difference between the app and using it in a browser. I imagine that it uses a web view for the UI. It is a little more convenient to have it as a separate, executable app rather than having to find the browser tab for it.
This week they announced that ChatGPT will be able to directly interact with some apps. I think the list is very limited so far, but that is something that the browser-based version will not be able to do.