They keep everything in balance.
They keep the best grazing areas from becoming overeaten.
And even help coral reefs!
But how do these things affect us?
In more ways than you might think!
No matter where we live in the world, half of our oxygen comes from the sea. And considering we're eliminating our other primary sources of oxygen (forests and trees) pretty quickly we should be VERY concerned about the effect a shark-less ocean.
There is no Planet B.