A traditional plaster surface constantly leaches calcium. This changes your water chemistry by raising your pH, and thus affects how long the sanitizer will last in your pool. Very well-balanced water with the proper pH and the proper alkalinity will stay balanced, that is, hold the free chlorine much longer than one that's bouncing all over the scale. Fiberglass is a stable surface, so it's not going to cause the water chemistry to flip flop. When you think about it, we're putting plastic on the inside of a pool.
Plastic is more heat retaining than cement.
So it saves energy costs. Furthermore, many pool managers chose fiberglass pools because they've had it with the stains plaster is prone to. A lot of people go to fiberglass for color or because they want to minimize surface staining. Any plaster pool over time will show staining. According to Dietz, who warrants any fiberglass liner he installs for fifteen years, a properly installed and maintained fiberglass liner should last a minimum of five years and more likely fifteen.