Sharks play an essential role in balancing the Food Chain
Typically, sharks will predate on the weaker prey, ones who might be older or ill. This leaves all the healthy animals to be able to live on and create the next generation of fish, squids and rays.
Without apex sharks in an ecosystem, there are no predators for the other carnivores. These carnivores are able to increase in population, since nothing is eating them. Food for these carnivores very quickly runs out, leaving the carnivores either hungry, or having to adapt to a different diet.