Can someone please explain me what's good about teaser websites?

Like the countdown for the square enix announcement...

People talk about it, sure.
But, if the secret is found before the announcement, then the announcement is kinda ruined, everyone knew.
And if the secret is not found, you've got hundred's of fanboy telling it's gonna be such a megaton that when you make your annoucement, it just sounds like "meh".

See metal gear for iphone. Everyone was claiming it's metal gear for 360 or a new one for PSP or for PS3, etc...

And when they said it was for iphone... mmmm ok...

So really? what's the point?

Speculation = hype.

They're overused, but when used correctly for a game that's actually a big deal it can work well.

I see no good point. I find them annoying and they are being done so much it just ruins the whole idea.

They are just always annoying, because 90% of the time they are disappointing, and there are much better ways to reveal these things