Without an inertial dampener onboard:
When a ship accelerated in space, the effect would be anything that isn’t bolted down would be hit with the body of the ship that was moving forward.
- All humans on board the vessel would likely be reduced to smears of paint.
- Star Destroyers are so large that they would likely just be torn asunder from the gravity well of a planet unless the craft itself was able to manipulate its gravity and those of the objects within.
The combination of inertial dampeners and repulsorlift technology allows a spacecraft to withstand the forces of entering and exiting a planet’s atmosphere. Even if you increased speed in space, it would only turn a Star Destroyer into an artificial meteor storm.