My favourite reason is that I can move retry logic into the thing sending the HTTP request and my front end web handler thing can be significantly less stateful as a result, which makes restarting it a lot less annoying etc. etc.

I'd probably have started out with -just- the outbox though and bolted the HTTP request sending onto the database, then looked at switching to a dedicated queueing system later. But that's a "because I already understand the care and feeding of PostgreSQL" choice as much as anything else.