They do that on 90 degree V6 engines to make the pistons hit TDC at even intervals. If you don't do that on a 90 degree V6 engine, you get the pistons hitting at uneven points during the crank rotation and the engine balance is wonky. You get either 90 or 150 degrees between pistons firing, depending on which ones. These are "odd-fire" engines. If you offset the journals 30 degrees from each other, you get an "even-fire" V6 engine where you get one piston firing for every 120 degrees of rotation of the crankshaft.
The Buick V6 that was sold to Kaiser for the Jeeps in the 60's and back to GM in the 70's was originally an odd fire V6. In 1977 GM made a different crank with offset journals turning the engine into an even-fire.
More info on odd vs even fire V6 engines here:
https://en.wikipedia.org/wiki/V6_engine ... ven_firing
However, if you have a V6 where the two banks of cylinders are 60 degrees apart, you can naturally make that an even fire with straight journals.
Offset cranks like that are used in other engines too, but the most common offset crank engine in production is the 90 degree V6 even fire.
