@AnushkaMu ,
Adobe Experience Platform is built API-first. So all UI features are built using APIs which you could also use. Functionally there is no difference between something built with the UI vs the API.
Because the API are built first, sometimes there may be periods of time when there is functionality available through an API for which a UI has not been built. For example, before the Sandbox tooling feature was built, the APIs were the only scalable copy things between sandboxes. Off the top of my head, I'm not sure at the present time of something you can do with the API that you can't also do through the UI, but perhaps others in the Community might know.
Developers can also build services leveraging the API, such as one of use cases for the flexible audience feature mentioned in this video: https://experienceleague.adobe.com/en/docs/platform-learn/tutorials/audiences/evaluate-audiences-on-...
