what are the benefits of good extension during the delivery?
Clarify what you mean by “extension”.
releasing the ball as far out infront ofthe body as possible. I know it helps with perceived velocity.
Ok, a release point that is out in front does increase perceived velocity which is part of a pitcher’s deception. It also tends to produce both better movement as well as later movement if for no other reason than the release point is closer to the batter.
But I wouldn’t think of “extension” as the way to get the release point out front. Instead, maintaining good posture and balance and having the timing to stay closed and rotate late are the things that help get the release point out front. Also, getting the head, shoulders and upper spine stacked upright coming out of shoulder rotation instead of immediately flexing the trunk forward will also let you get the release point further out front. This last point is often more of a strength issue (in the low back) than a mechanics issue.