I'm the same, been wondering what that is since day one.
I can only apply computer terminology & think it's some kind of heatsink or something to dissipate heat, like what you see sitting on top of CPUs & GPUs thesedays...
As far as I remember it is also a fan speed controller. Our fans don't just go on and off but the speed is varied depending on what the engine requires.
with my old galant, when i upgraded the radiator into a 7g vr4 type, it came with that too.. but since i couldn't connect that to the factory harness that my car came with i didnt connect it.. so i took it off but the fan just kept running.. but when i connected it some how it ran as normal (turned on when its supposed to and turn off when not needed - even with the A/C)