Noone would ask this question about humans. Humans use tools, coordinate their activity by communicating detailed questions, statements, and commands, and contrive elaborate schemes for altering their environments to be more favorable. The ability to do these things is accepted without controversy as being essential to their success as a species.

But dolphins: what do they do in their natural environment that utilizes such intelligence? If they just swim around eating fish and mating, then they need not be any smarter than sharks, which do basically the same thing.