It’s leverage. Fulcrum and lever stuff. The further out towards the barrel that the bipod is, the less that a specific movement on the butt stock will affect barrel aim. As with most things, there’s a balance though. Too far out on the barrel, such as the very end as an example, and the shooter will have to move too much to shift the point of impact during dynamic situations.
The simple answer is: to reduce excess barrel movement for accuracy, but too far forward can be a negative.