Our city does offer its own festival of Geek culture, but we honestly feel ours is better, for a few reasons. First, ours is free while the city sponsored event costs $12 per person for a one day convention.
Yes, $12 is cheap when compared to the big, big conventions, like San Diego (up to $66 dollars per day and nearly impossible to get), Wonder Con (about the same as San Diego) or even the Phoenix one (our state capital, which is actually pretty "cheap" at $75 for the whole weekend) but when you consider that our county has one of the highest unemployment rates in the country, and many households have multiple children, that's pretty cost prohibitive for families with kids.
Second, community feedback tells us that the city convention has been declining in "bang for your buck" recently. Up until last year, the convention had been planned and run by city employees, including a couple of people who have worked for our library district before or since. They were able to bring a pretty good mix of vendors, panels and entertainment to the event. Actually, the first two years the city convention was held, our library participated with panels and booths. In fact, the last year we participated we provided the event its entire complement of child-appropriate attractions. As of last year though, the city contracted the convention out to a third party and the decline was immediate. Our governing board decided that our library could no longer participate in any event that charged an admissions fee, even if it was something as small as $1. This took us out of participating in the city convention, as well as our local Renaissance Faire and a variety of other events we had participated in previously.
Consequently, the city convention lost just about all of its child-friendly components. Feedback after last year's event was that there were very few panels, and the convention mostly consisted of a lot of vendors and people milling around in Cosplay taking pictures with not much else to do.
Finally, considering that this thing is basically planned by two people (myself and one of our reference Librarians) we have a much wider variety of programming to offer than the city convention. Offerings at our convention have included a Cosplay Contest (virtually mandatory for a convention of this type no matter WHERE it is held), guest authors, guest artists, Virtual Reality demonstrations, 3D printing and other technology demonstrations, children's story times led by "special guest" characters, skits, trivia contests, art shows, crafts and more.
So basically, the reason we host a convention (aside from showcasing our beautiful facility and all the cool stuff we offer our community) is that our community really wants and needs a free, fun, all-day geekfest for the entire family that gives them an opportunity to experience the library in a completely new and interesting way. Even the people who wander in not knowing the event is happening tell us they are amazed at how cool and interesting it is, and how glad they are they stumbled upon it.
But mostly, we do it because we're geeks. We're proud to be geeks, and GEEKS RULE ALL!