Date Bar display date and day on the center of top status bar. When in the middle of working something that need date Date Bar give convenience by displaying date directly without having to switch to calendar app. Same with when you forget what day is today simply look at Date Bar.

Date Bar is auto start everytime device turned on for convenience so you don't have to manually run it. Some devices have cutout (ie: for camera placement) on position where Date Bar display date. Date Bar won't be overridden by cutout and will automatically display below it.