Those of us who sail in the San Francisco Bay often see 20-30kts every summer day. It's been my experience that if you don't ease the main immediately after jibing, the boat will round-up (broach). If one thinks about the boom as a giant lever arm connected to the boat's z-axis (parallel to the mast) and the wind is the force at the center of effort (the sail), then keeping the main in enables the strong wind to spin the boat quickly.
By the time the boom is nearing the shrouds, the pressure on the sail and the turning moment are reduced significantly so that the load on the main sheet and the boom are much less than if it were left sheeted in.
Forgive this explanation from an electrical engineer.. I'm sure someone more mechanical can explain it better.