1 - It looks hot. 2 - It's sensual - both not being able to scream and to not be able to communicate 3 - It's considerate. Can't have the girlfriend screaming and disturbing the neighbors
I've found a way that makes people feel rather helpless and somewhat creeped, but not in a bad way. Only blindfold them, then be quiet around them. Above all, don't respond to anything they say. If I ever find myself without enough things to gag AND blindfold, I just blindfold. Try it out, it's fun.