its fairly standard for all mill wood (not necessarily needed for pulp) if its hand fell and limbed, then likely one side will still have stobs and lumps that were missed. So the logs get rolled so the stubs can be knocked off.
its more to do with our type of debarkers then anything, ring type that encircle the log, then the log is shoved through, big stobs and lumps will hang up on the mechanism and cause all sorts of trouble, so either the poor SOB that runs the debarker has to bump them, or its done after grading.
Because of this it can potentially get the log culled, one or two stobs isn't a big deal usually, but if one whole side of the log is covered in 4" long 4" dia stobs... they mill gets upset in a hurry. One of the buyers at one of my favorite mills, used to run their debarker, (its a pretty skookum show that builds folks up from pushing brooms to potentially being CEO, its employee owned and operated) anyway every step of production the guys working there worked at the position before and so on....anyanyway...the buyer is younger then me, and he's super open about what the mill needs and wants, just a few minutes talking to him will change your perspective on what makes a good log better, or worse.
For mechanized processing, its pretty much a non issue.
the other benefit of bumping knots is that the logs will sit better on the truck, meaning the driver won't have to stop 15 times on the way to the mill to tighten wrappers, and for stuff like cedar you can maybe even get 1-2 more logs on before you're over height