Regulatory (legal) obligation.
CRTC required wireless carriers to keep customers informed of the CRTC Wireless Code (customers have codified rights, where customers can go for information, how and where customers can report complaints). CCTS Wireless Code likely inherited this requirement.
I think the carriers operate whatever schedules they like for these announcements. They might even "voluntarily" do send more messages than the absolute minimum required of them, just to ensure they stand on solid legal footing. They might arbitrarily broadcast reminders whenever sensitive/controversial Wireless Code topics gain visibility.
