We recommend registering a separate representative of the advertiser or agency for working in an app. If you are using the same username in the web interface and the app, you need to change the password often for security reasons. Changing the password means you have to get a new OAuth token. If the username is only used in the app and not in the browser, there is minimal risk of it being compromised, and the password can be changed less frequently than you normally would.
It is particularly useful to create a separate representative if the app doesn't interact with the user (for example, a script updates bids automatically). In this case, it is difficult for the app to get a new token.
If you use several different apps for working with Yandex.Direct (such as apps that perform different tasks or were created by different developers), we recommend creating a separate representative for each app. If you encounter problems in one of the apps (for example, the token was leaked or too many points were spent), you will just need to revoke the token for this representative.