Yes, you can cover it up with plywood. However:
was made by builder for no reason
No, it was for a reason. If it was for no reason, they would have likely just covered it with regular siding. When they went to the extra effort of a proper cover, that's an indication they really intended to make the hole. You just don't know what that reason is. Typically, this would be a vent for a dryer, cooktop or bathroom exhaust. Either there is something typically included in the house that wasn't included here, something that was removed or something that was never installed (that should have been installed).
A plywood cover with a few screws is easy enough to remove if you later figure it out. But consider all of the following:
- Do you have a clothes dryer? Is it vented? Is it possible that some owners put it in one location and some put it in a different location?
- Kitchen cooktop - Does it currently have a fan? If not, is this close enough that it could be used for an exhaust fan?
- Bathrooms - Bathrooms generally require a fan except if they have an operable window of a certain minimum size (specified in the building code). Is there a bathroom near this location that doesn't have a fan but should? Or that doesn't have a fan because it has a window but could have a fan connected here?