When Blofeld leaves the control center in You Only Live Twice, he gives the keys to self-destruct to his bodyguard, Hans.
Why did he do that, as opposed to just throwing the key out into the piranha pool?
Well, Blofeld gave Hans, who was his trusted henchman, keys to self-destruct to blow up the spaceship.
When Blofeld gave him keys to self-destruct, there wasn't piranha pool. However, why would he ask him (Hans) to throw the keys when he wanted to destroy the spaceship once they captured the space shuttle.
Here's a snippet of conversation between Blofeld and Hans.
Blofeld: Hans, our job will soon be done. Blow them up as soon as they have captured the Americans.
Here is the key to operate the exploder button.
That's here Bond interrupts their plan and destroys the spaceship before it could capture the American space shuttle.