Date overrides are a feature that lets you add one-off adjustments to an availability schedule. This is useful when you know that your schedule might be different for a week or two but you don't want to create a brand new schedule just because of that.
The date override feature lets you specify the day and hours when your schedule is different. An example of this might be if you normally work Monday to Friday but, in order to cover for a colleague who's on vacation, you'll be working Tuesday to Saturday for the next two weeks.
To summarize, instead of creating a brand new schedule and then editing every event that references your old schedule, you can just use the date override feature instead.