A broadcast is a single outbound communication that is generally best used for a Time Sensitive communications or Date specific communications.
Your campaign is built to deliver messages over time, and to deliver those in a pre-defined sequence from when the person opted in. The broadcast can be used to alert customers of a special offer or time specific message.
For example you have a campaign for a Florist shop. You have a list of people who have opted in over time and receive your pre-determined follow-up sequence.
Today you're having a Valentines Day Special. Here you can use a Broadcast so it reaches everyone in the campaign, no matter where they are in the timed sequence.