This PPRuNe thread includes some debate on the subject from 20(!) years ago:
737's crabbing on the ground
Appropriately, given today's date, it includes some droll contributions. I particularly enjoyed the explanation that the aircraft crabs when taxying because of the torque reaction from the rotating anti-collision beacon, as well as the caution that due to the Coriolis Effect it was extremely dangerous to taxy across the Equator.
I suspect that the shimmy damper explanation is closest to the truth. It's hard to believe that gear is free to castor in the sense of completely unrestrained rotation.
Apropos of the latter, I can vouch for the fact that the Shorts commuters were able, with a bit of sporty manoeuvring on the ground, to run the nosewheel steering pinion completely off the rack and thereby get the gear to rotate 180°.