Hi @rkbnair 
To make requests to Shopify APIs, they will need API credentials, including an access token. If the app uses OAuth (i.e., public or custom apps created from a partner dashboard), the app will request a token upon installation and there is no need to create a separate custom app. Otherwise, to access to Shopify APIs, a custom app will need to be created so that appropriate API credentials can be generated.
Hope that helps!