As another commenter said, "to have an art gallery," but also because you don't know which of the (imaginary five) might be your best collector in five years.
A big part of running an art gallery is being available for people who want to look at art. Some of those buy, some of them spread the word.
In the end a small number of people are going to keep your gallery alive, if it stays alive. But you have no way of predicting who they will be or whence they will wander into your space. That VP of Sales in town for a conference who only has Wednesday afternoon free and is starting a collection of $YOUR_SPECIALTY.