The workers have to poll Postgres repeatedly at intervals, whereas a worker using a message broker (like RabbitMQ) connects and just waits for something to get tossed its way. Having your task broker separate from your database can save a lot of DB IO if you've got a larger number of workers or a busy task queue. RabbitMQ (and the other more full-featured message brokers) can also do a lot of really slick things with routing, prioritization, and all sorts of other goodies.

Of course, this doesn't matter if you're running a small site with low traffic. I wouldn't get too caught up in worrying about broker selection unless you're cranking out a lot of jobs, or have special needs.