Let me explain...
We use both, passthru-auth and ldap-auth, and combined it (which is possible without any modification to the plugins) to get our users logged in via SSO (apache + AD).
First we've set up ldap auth plugin to work like expected. After that we added the auth-passthru plugin by downloading the files from the github repo and building the phar with php - so we are sure client passthru auth is working (currently the auth-passthru plugin downloadable under www.osticket.com/download includes only passthru-auth for staff). We especially added the auth-passthru so that the users/staff get automatically signed in via SSO using IE, Chrome or Firefox (for firefox we added a special group policy to enable SSO for specific webservers - it's not enabled by default, but using http://kb.mozillazine.org/Locking_preferences with the right preferences it can be enabled domain-wide ;) ).
Since the web-server is doing the authentication, the people just open the ticket website (e.g. http://example.com/osticket) click on "New Ticket" or "Check ticket status" and do not need to enter anything - they are signed in right away by the webserver and can instantly start by viewing their tickets / creating new tickets.
Same for staff. Staff just opens the scp page (e.g. http://example.com/osticket/scp) and is instantly signed in without the need for entering username or password again since the http-passthru auth matches AD account to the staff account and then you are signed in. In case an AD account is not in the staff, they will just see the normal scp auth page and can still login their with a local osticket account.
So we basically use it to make it easier for our users to get signed in to the ticket system (either client or staff page). Hope you now understand why we use it ;) Btw: we use osTicket only internally (which made the decision to use passthru-auth / SSO really easy)