Why did Snape teach Occlumency?

[demetra1225]

No: HPFGUIDX 103912

--- In HPforGrownups at yahoogroups.com, "dnp1234" <davyval at c...> wrote:
> Voldemort must think that Snape has Dumbledore's confindence if he 
is 
> a teacher. Snape could easily explain to Voldemort that he is 
> purposely making Harry fail with his lessons.

Demetra:
But that scenario would mean that Voldemort knows that Snape is an 
Occlumens.  In my mind, the only way Snape can be spying is if 
Voldemort has no clue that Snape is an Occlumens.  
I think Sonja was asking why let Snape teach Harry occlumency because 
then Voldemort would find out that Snape is an Occlumens and has been 
lying to him.  If Voldemort was able to perform Legilimency on Harry, 
I would ask the same thing.  
But the link between Harry and Voldemort is not one in which 
Voldemort "reads his mind".  As far as we know through the link Harry 
has sensed Voldemort's emotions, Harry has seen Voldemort's 
activities in real time (the Snake attacking Arthur) and Voldemort 
has placed a vision in Harry's head.  No evidence that Voldemort has 
performed Legilimency on Harry except for perhaps the one time in 
PS/SS when he surmised that "He lies."  But they were face to face, 
this was not done through the scar link.  
I guess this brings me back to my question about the whole thing.  
Why was it so important for Harry to learn Occlumency, when what he 
needed to block was not Legilimency?