Pub/sub suits one event with several consumers. When an order is placed, the warehouse, the invoicing system and the email service can each react without the order system calling them one by one. Adding a fourth consumer later means subscribing it, with no change to the sender.
The common confusion is with a queue, where each message goes to one worker. Many designs combine the two: Amazon SNS publishes to a topic, and each subscriber reads from its own SQS queue, so one slow or offline system does not affect the others. Consumers should expect occasional duplicates, because most of these services deliver at least once.