Why naval shells need explosive fillers

Are explosives in naval shells necessary when the projectile already strikes with enormous kinetic energy?

Yes; impact can penetrate, but the explosive filler spreads damage through the ship and makes a disabling or sinking hit more likely.

A fast, heavy projectile can produce severe local damage on impact. Its explosive filler remains important because detonation after penetration attacks a much larger internal volume, disrupting machinery, structural connections, and systems beyond the shell path. A penetrating hit without that secondary effect may wound a ship without delivering the decisive damage sought. Armour penetration and the internal destructive mechanism that turns a hit into a kill are separate effects.