By doing this no one(in the Harry Potter world) would dare question his allegence to Lord Voldemort.I also think it's interesting when in the departure Snape simply countered Harry's attempts, without really doing anything to really injure Harry.
Not anything permenant that would make him easy pickings.
No semi-permanent attacks. . .Nothing really.
"Blocked again, and again, untill you learn to keep your mouth shut and your mind closed, Potter!"
advice to Harry on the art of occlumency hidden in a taunt.