The main value of these systems is normally not "relative queuing" (although that is sometimes done). Often, there are certain activities you might want to do

The main value of these systems is normally not "relative queuing" (although that is sometimes done). Often, there are certain activities you might want to do in a way that doesn't block the request/response cycle, but still have them performed as soon as possible. For example, a user buys a widget on your site - send a receipt email. You don't want the "Thanks for buying" success page response to wait for your servers to compose & send the email, so you offload the email to your queue system.