Usefulness of graduate school extracurriculars
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.