As a home inspector in St Louis, I come across attic fans fairly regularly. The idea is to open windows on the shady or otherwise cool side of a home, and pull the cool air in, while pushing the warmer air up, and out of the house and in turn, attic. If you like fresh air, and the units louvers seal well when turned off, it's great, but know, that they are generally poor insulators in their normally off mode.

If it's not for you, though, it can be removed and sealed off completely, or even just temporarily, in which case it would be beneficial to insulate over it, because, as stated, they are quite prone to heat loss in the winter. Have the switch removed, as well, to prevent accidental switching on while covered.