Larson spring gages measure spring free-length as the springs are manufactured and adjusts the spring pitch during production. Spring gaging produces parts with a more consistent free-length. Out-of-spec parts are sorted out and rejected during production.
You should gage to reduce and eliminate scrag at the point of manufacture. Your spring coiler does not always produce the perfect spring, even if properly setup. Spring wire often varies in diameter and tensile strength, which affects the pitch of the resulting coil. Pitch control tooling is subject to wear, which can affect pitch as well.