Usefulness of graduate school extracurriculars All graduate programs have committees such as the "social committee" and "speaker series committees", as well as Graduate Student Organizations and other organizations to work on behalf of these programs. Is there any concrete career benefit to being an officer on one of these committees/organizations?
Those volunteer positions may help boost your chances of getting a major award.  For example, one of the most prestigious graduate scholarships you can get in Canada is the Vanier Scholarship and the selection board uses your leadership experience as criteria for the award.  

In addition to scholarships, it's always nice to have additional things to add to your CV to make you stand out.  These skills can show a number of traits that employers may look for.