It's worth noting that nowadays 0day is generally looked down upon; there are now 0hour, 0min and, yes, 0sec couriers, in that hierarchy. 0day is generally reserved for the lowest of the low; anything more is not for couriers at all, but the general downloading public.
The amount of time in this hierarchy is not the amount of time it takes to get to your hard drive - it's the amount of time from release before it is on a site that you have access to. If you are a 0sec courier, you have access to release sites, i.e. the sites where the software is first uploaded to after being cracked.
Each group (Razor1911, Paradigm, etc) is generally affiliated with one or two sites. From these sites it generally goes (via the aforementioned 0sec couriers) to all the other release sites and very few non-release sites in 0min (so being a 0min courier usually means you're 0sec somewhere, too). From these sites it generally goes to the next level in the hierarchy, 0hour, and eventually it makes it to the 0day sites, after which the general downloading public gets it.
So, as you can see, couriers are the veins and arteries of the warez system. Why do they do it? Every site works on a ratio basis: you upload x bytes, you can download y bytes (usually y=3x). Couriers have programs with FXP capability: they can transfer directly from site-to-site. As these sites are very fast, the easiest method of accumulating credits,
then, is to "curry" (I kid you not, the new verb form of courier is curry) software from one site to another, rack up credits, and then when something really cool comes out you can send it off to a dump site and download it at your leisure. Also, the sites publish lists of top uploaders and downloaders - there's a very large element of competition involved.
Disclaimer: The information contained herein comes from my memory of the sc3n3 as it was 2 or 3 years ago. I don't think very much has changed, other than the fact that both RZR and PDM have been busted and cracks have become increasingly irrelevant as more people are able to download ISOs.