What's the difference between rectangular tea bags and pyramid tea bags?
The pyramid teabag allows for more effective and faster extraction. This is because the tea bag retains its 3 dimensional shape, so there is plenty of space for the tea leaves, and more of their surface area is exposed for extraction.

A conventional teabag actually becomes smaller, and there is less space and the tea leaves are pressed onto each other, creating a compressed little ball of tea. I think whoever invented the pyramid teabag had that in mind. I am not suggesting that the world was desperate for it though ;)