I think a hard part with lots of these “what do I use x for” examples is it starts with the tool and the discusses the problem that it solves. I find it more helpful to start with a problem, and discuss the various tools that address it, in different ways.

Forget email, say you have a app that scans links in comments for maliciousness. You rely on an internal api for checking against a known blacklist, which follows shortened links first, and an external api from a third party. You want the comment to appear to submit instantly to the poster but are comfortable waiting for it to appear for everyone else. What are your options?

You could certainly use message queues and workers. If you’re cloud native maybe you leverage lambdas. Maybe you spin up an independent service that does the processing and inserting into the database in the background, and all you need to do is send a simple HTTP request on an internal network.

Your solution depends on your throughout requirements, the size of your team and their engineering capabilities, what existing solutions you have in place. Everything has its pros and cons. Pretending that celery/redis is useless and would be solved if everyone just used Java ignores the fact that celery and redis are widely popular and drive many successful applications and use cases.