"Needed"? The rocket has maneuverability without them, sure. Just as your car has maneuverability without power steering, and your car has braking capability without ABS. But the grid fins increase the maneuverability a great deal at low cost, which is why they are used. They make it far more likely that the rocket can successfully hit a landing spot during a re-entry. So even though they add some weight and add some cost and add some complexity to the system they still increase the chances of recovering the multi-million dollar rocket hardware, which is a big win.

And the hydraulic fluid running out affects the impact velocity because the terminal trajectory of the rocket was controlled based on the assumption of having the grid fins working. When they stopped working they put the vehicle in a situation that was no longer controllable given the remaining capabilities of the rocket. If the power steering goes out on your car in the middle of a turn that's much worse than if you didn't have power steering at all, and could result in a crash.