I have a couple of ideas as to what these are there for, but I have vents in
my ceilings that go to nowhere. They are in all the rooms, all of them go
into the attic and are all connected to each other by regular ducting. But
they don't exhaust anywhere.

I'm not exactly sure why they exist.

I'm thinking of two possibilities
1) They are from an aborted attempt at installing AC
2) They are for the purpose of dumping hot air into the attic.

I'd always assumed that they all connected up with each other and routed
back to the furnace, but the ducting literally goes nowhere - it's all
contained in the attic and it's all SEALED - meaning there is no exit port.

What got me wondering about it was whether or not I should seal the vents
during the winter to conserve heat.