Of course there is one reason to use Ticketmaster: you often don't get a choice anymore. The Venue box office is a joke, and Ticketmaster has an exclusive agreement for the rest so it's either them or pay even more to a scalper.
This is why I go to barely any shows anymore. The ones I do go to are at the one local venue that doesn't use Ticketmaster (Wolftrap).
It is a testament to our weak antitrust laws that Ticketmaster has been able to gouge the public for decades with no repercussions whatsoever.