What are the advantages? Why not just a directory like
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I see at least three possible (good) reasons:
That's what is done on StackOverflow if I remember correctly
The primary reason I think, is for cookies.
Cookies will be sent along with every request, let's say if you have 2kb of cookie data and load 20 images on a page.
That's an extra 40kb of data, multiply that by the number of page-views you have in a month, and you might be surprised about how much bandwidth you have lost in something useless, and bandwidth is not free...
Also, cookies set on the top-level domain, are sent across all requests made to any subdomain, in those cases, is even recommended to buy a new domain to host the static components there, in a cookie-free domain.
For example, StackOverflow uses
sstatic.net, Yahoo uses
yimg.com, YouTube uses
ytimg.com, Amazon uses
Give a look to this:
... use cookie-free domains for components.
When the browser makes a request for a static image and sends cookies together with the request, the server doesn’t have any use for those cookies. So they only create network traffic for no good reason. You should make sure static components are requested with cookie-free requests. Create a subdomain and host all your static components there.
This advise was taken from Yahoo's Best Practices for Speeding Up Your Web Site.