WHMCS added the ability to ‘watch’ tickets in a while back, but this has to be enabled manually per ticket. What a hassle
With this addon, you can have this automatically added, and when the ticket is closed, the ticket can be unwatched automatically as well (not recommended)
With WHMCS Support Extended, you can automatically assign a ticket to a user when they reply. This is not done by default, or able to be done in WHMCS without an addon
On closing a ticket, you can have the custom fields of this ticket wiped out entirely. This is recommended for secure installations.
You can have the custom fields of tickets encrypted . This is recommended when storing sensitive data.