Read the link given by Tiny - it is a good read.
Babbitted rods have the bearings as an integral part of rods. Ultimately, when the clearance between the "bearing" surface and the crankshaft gets too large, you have to replace the rods (to get new babbitted bearing surfaces ($$). If the rod-bearing journals on the crankshaft have to be ground, you need new rods/babbitts. There are also adjustment issues that required care and skill (and, luck).
Precision insert rod bearings allows bearings to be replaced when they wear, and allows rod bearings to be replaced if the crankshaft has to be ground due to wear.
