A wider assortment of cheaper bearings is available. Smaller journal gives slower bearing speed(desireable at HIGH RPM). Reduced diameter allows stroking for more cubic inches. Weight reduction at throws MAY eliminate the need for heavy metal(expensive) when balancing.
NHRA allows using a PONTIAC rod journal size, (2.250"), so an aftermarket Oliver Pontiac rod can be used. That rod is 6.625" long and most companies sell it either bushed of press-fit for a lighter .980" Pontiac wrist pin.
Most of the advantage is rotating weight but, there is some gains in having less bearing surface speed.
Edit: The NEW engine specs listed by NHRA for this year's "Stock/Super Stock" combinations now allow a 2.200" Chevy journal to be used as long as you also use the NHRA approved connecting rods, (and ONLY the approved rods), in the engine.
They, NHRA), now also allow the use of Cleveland size main journals as long as spacer shells are used in the block in order to use the Cleveland, (or B.B.Chevy), size main bearings.
NO aftermarket block having smaller main bearing specifications.