From Wikipedia: Sign-on and sign-off sequences have become less common due to the increasing prevalence of 24-hour-a-day, seven-day-a-week broadcasting. However, some national broadcasters continue the practice; particularly those in countries with limited broadcast coverage. Stations may also sometimes close for transmitter maintenance, or to allow another station to broadcast on the same channel space.
Learn more: https://en.wikipedia.org/wiki/Sign-on_and_sign-off