You can program them how often to exercise and when to exercise. Depending on the switch gear you can do loaded run time as well. I'm not sure if generac was/is offering it, but quite a few manufacturers started installing small load banks in their diesel generators to deal with aftertreatment issues from being light loaded all the time.
It was actually one of the biggest issues we saw with generarors. The older diesels would never fully get up to temp and slobber like crazy plugging the exhaust manifold, piping and muffler. Normally tossing a load bank on them and giving them a solid load for a few hours would keep them cleaned out. Seen more then a few to the point where the exhaust caught fire internally from all the soot and snott built up. The newer engines have the same issues, just worse with all the emissions equipment. They can't be left to run for long periods of time for the same reasons, however they cost a lot more to fix then just loading them up. I know kohler, power secure, and multiquip all started installing them. I'm 90% sure the generac we had at the township also had a built in load bank, but it was also ran under load once a month.
Really the same applies to any liquid cooled engine, they just don't get worked very hard sitting there doing nothing. Another issue is everyone wants to spec a generator thats too big for their needs. You really want to see at least 60%load in use at any given time, really we liked to see an 80% load but that wasn't practical in some installations.