The purpose of the safety joint is to enable the inner barrel to be extracted from the core barrel if the outer barrel becomes stuck in the hole.  It also provides a convenient separation of the outer barrel and inner barrel for servicing.  The adjustable safety joint facilitates the spacing of the inner barrel.  This saves rig time since spacing can be achieved without breaking out the swivel assembly, which must be broken out to adjust the inner barrel spacing with the use of shims.  The adjustable safety joint offers more accurate spacing of the inner barrel.