Dramatic effect, mostly. And the technology behind the scenes improving.
The shield bubbles of TNG were cheap and affordable in he late 80's/early 90's. They couldnt go repainting and damaging the models every time a ship took a hit. With late 90's CG, thats easy. So they dropped the bubbles and showed us the far more visceral sight of gigantic starships carving each other up with death rays.