Here are some advantages of the addon domain -
1. Main advantage is that the addon domain used to share the same control panel of your main domain so it will reduce the cost of ordering the new hoting service for it.
2. Addon domain is the domain on which you can create the separate site which will just share the same control panel but will have it's own email accounts, FTP account, directory assigned, databases.
3. It will share the resources, bandwidth of the main account.
4. You can create the backup of addon domain too.
5. In future, if you would like to move the addon domain to it's own site then you can simply need to provide the database name to other provider and login details of panel, there is tool available which converts the addon domain to it's own account.
6. Addon domain will have all the security provided as of your main account.