I have a relative in the hospitality business (books conventions and groups for large hotels and facilities). I never really understood before talking to her about how much time, money and effort goes into booking a site for something like a Regional Convention or the like. The logistics of making it all happen in an orderly way is much greater than most people think. That is why there are people who do this for a living and get paid a lot to do it well.
And the Society tends to want things done "their way" instead of more involvement by professionals to save some money. Depending on the location, it can involve various types of insurances, traffic permits, parking issues, security, facilities setup and dismantling, negotiations with unions for work to be done by volunteers instead of union employees, etc, etc, etc. A lot more behind the scenes stuff than most people realize.
In many cases, it really does make sense to use a building that may otherwise be empty instead of going elsewhere. Yes, there is some wear and tear on it but the net cost is typically lower than renting a commercial building. And being in one of your own buildings cuts down on a lot of the red tape and scheduling requirements that come with a commercial building.
Other than the visual impact of being in a large convention with thousands of others, the smaller ones are a lot easier to control and manage and be cost effective.
Rub a Dub