The impression I get is that it's for things that can be done asynchronously -- that don't need to be completed immediately in order for the workflow the user is doing, or that might take longer than the 5-10 seconds a user is generally willing to wait without reloading a page.

From a user experience standpoint, and completely ignoring the technical and of things, anything that might take more than a few seconds is going to lead to people thinking that something's not right and hitting Refresh to try again, which will result in multiple jobs, etc. As far as I'm concerned, anything that might take more than 5-10 seconds on an average account (ie, not someone like scans_daily with their 5000 tags) should be done asynchronously with the user notifed when it's done...