You can change your password in the Security page of your
Settings Menu.
You can view the connected device to your Nextcloud account and perform action such as Revoke session or Wipe device (if supported).
The “Create new app password” is for creating specific “application access password” for various access App (Nextcloud Desktop, Nextcloud Mobile…) if you are using Two Factor Authentication (2FA).
Even if you are not using 2FA, it is still a good practice to have app password created for various App so that you do not use your main password on individual App for security reason. An app password cannot be used to login via Web browser interface, which provides ability to change settings.