What is the need for a 110 GT score? Why is it important?

A 110 is pretty easy to get with some studying and following the guidelines of BSEP course.
A 110 doesn't do anything for you. A better question is what requires a 110? Commissioning, several job positions, broadening assignments, and units require a 110 GT, and it's not waivable. But if you don't plan on commissioning, or applying to a special duty that requires a certain GT, then it has no effect on you.

Pretty much every job worth having requires a 110 GT. That's about as straightforward as you'll get.

It's a mark on a yardstick.

The military figures if you can hit certain benchmarks, you might do well in certain roles. It's no guarantee that any given service member with a high GT score will equate to excellence in whatever role requires a high score, but they probably have corelating evidence that shows one is more likely to do well if they can earn a high score; hence the benchmark.