What is the significance of allowing the usage of combined shaft resistance and end bearing of rock socket to resist loads?

In some design codes, it allows the simultaneous usage of shaft and base resistance of rock sockets to resist loading. Past experience showed that the shaft resistance could be mobilized in rock socket provided that the length of rock socket is less than three times the pile diameter. Such provision eliminates the need for bell-out because higher pile capacity could be derived from the simultaneous usage of shaft and base resistance of rock sockets. Bell-outs are commonly used in piling industry but its effectiveness is doubtful because of some practical issues like the inability of thorough cleaning of pile base.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.