It depends. What is your use case? If you mean this project specifically, part one says "you will probably never need to build something like this yourself (nor should you)".

Dumping everything into an S3 bucket may be perfectly fine and even efficient for some projects. I personally don't see myself wanting to use S3 buckets for transmitting messages between multiple different systems with different purposes.

If you have a lot of different services producing data that multiple services will consume, then a pub/sub system can simplify things. Some systems, like Kafka, also have added redundancy/scaling benefits.