I agree with Jake and Yurdle. The dolphin striker post takes the full downward force of the mast. This force is transfered to a tensile force within the striker rod which is counterd by an axial compressive force along the main beam. With the dolphin striker securely fastened to the main beam and properly tensioned, the main beam should see no bending load due to the downforce of the mast (only compression along it's length due to the force of the striker rod pulling inward). The compression tube is there simply to prevent the user from over-tightening the dolphin striker post bolts and accidentally crushing the main beam. Case-in-point, boats like Hobie 16's & 18's (which don't use a double-nut sriker post) don't require a compression tube.

The compression tube in your boat is most likely stainless steel in order to prevent it from corroding and seizing to the dolphin striker post.

sm