I recently purchased a house with a ventless fireplace. It is a millivolt system that operates off of a switch on the log holder. There is a light switch mounted on the brick wall beside the fireplace that did nothing, so I pulled the cover off and found the wires not hooked up to the switch. The neutral and ground wires are spliced together but the black wire is taped up. Anyone have any idea what this switch may be for?

Depending on the model fireplace you have will determine if this switch is needed or not.
It can there to provde a few things.
One. its a remote switch to turn on and off the fireplace so you do not have to bend down to turn on. This is the most likely thing.
It can be a swtich for a future fan that is often supplied with these types of fireplaces. Its a way of turning it on and off. The installer may have provided this wire in case the owner wanted to have this fan put in at a later date.
Do not connect these wires until you have found the other end of the wire installed in the lower section of the fireplace. The grill near the floor will open up on a hinge which will provide access to the gas valve and all the controls to the unit. Once you have that access you should be able to determie what and where the wires goes. A wire diagram should be fastend to this area somehere as well. You will see this wire shown on this diagram telling you what ist for.

It sounds likely that the switch was originally installed for another use, lighting perhaps. If the black wire is taped up then it is not powering anything and the common and ground wires may be in series with other switches or outlets. That should be easy to check. I think I would put a wire nut on the black and replace the switch with a blank cover, perhaps painted to blend with the brick.