Perhaps you want to schedule a meeting on the 15th of each month. However, you don’t want the meeting to fall on a weekend day.
Or you want to schedule time to work on a biweekly report, every other Wednesday. But there are a few weeks when you are traveling on a Wednesday, so you need to reschedule those instances.
Here's a quick way to make exceptions for events in a repeating series:
Create a recurring event with the recurrence pattern you need, e.g. repeating on the 15th day of each month.