Probably to make you think you spent your money wisely... or to make you think it would be worth spending your money on. Back in the 80's people were complete dipshits (unlike these days where everyone is smart) and were impressed by things that were big. People would see the TurboBooster and think "Wow! There must be some awesome technology in that thing. Look how big it is!!!!" Then they'd spend the outrageous price that NEC charged for it.
In reality, the thing didn't need to be much bigger than the actual expansion slot at all (which is much smaller than the cover itself) and it didn't need to cost more than, say, $9. The Turbo and its accessories were a rip-off when they were new.