FWIW, Delta used to require F/Os and S/Os to get a first-class once a year, even though it was OK to let it "lapse" into a second-class after six months. (Don't know if this is still the policy.) The reason they gave was that it enabled them to obtain "certain waivers", perhaps having to do with insurance.

Also FWIW, a senior FAA flight surgeon advised that it's a good precaution to keep your first-class current, even if you only need a second-class now but will need a first-class down the road. He said that if some medical issue were to make granting a first-class a close call, they are more likely to renew an existing first-class than to "upgrade" a second to a first. If true, the reason for this may be more bureaucratic than medical.