What is use of external rotation of humerus in kocher's maneuver?

Answer 1

When you externally rotate the humerus, the superior glenohumeral ligament is no longer resisting reduction, and it puts the right part of the humeral head in proximity to the glenoid fossa.

Answer 2

External rotation of the humerus in Kocher's maneuver helps to reduce the risk of damaging the axillary nerve during shoulder reduction. This rotation maneuver helps to realign the humeral head in the glenoid socket by avoiding impingement of the nerve.