You gain much more experience wi fully upgraded development. If you want to make the best possible player then scout players that have decent stats, level 3 development and they will become awesome and gain experience incredibly fast.
You can have great players that aren't superstar development but all my 99 OVR are due to it and I had a 99 OVR QB that was 22 years old with max AWR, THP and max for all accuracy due to development.
It also increase the amount of XP you get from practices. The tied at 3rd quarter one was 500 XP for Normal, and 1500 XP for Quick. Am sure even better if upgrade to Superstar.
Yeah it's basically a multiplier you get 1x, 2x or 3x experience.