The American Board of Internal Medicine thinks it's important, so, to that end, (among other places) Duke Internal Medicine residents are required to do 10 hours per year of volunteering (it's certainly encouraged, but, at the end of the day, it's required).
On the other hand, in the medical community I'm joining, there is a free clinic manned by volunteers from all disciplines, so the time needed varies by the specialist (like, 4 hours for the dermatologist, and 1 hour for the EM and FM) - that is not a knock on those specialities - the extra time needed for these other specialities is often a benefit for the patients, so everyone wins.
My contract requires administrative duties, but the president says (rightly) that we are required to "do something you like", as people required to do administrative tasks that their hearts are not into do those jobs poorly.
So, to summarize this long-winded post, sometimes it's required, but, also, as DropKick Murphy states, maybe people like to do it.