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 images-amazon.com, etc...

Give a look to this:

• Use Cookie-Free Domains for Components

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 images-amazon.com, etc.

Give a look to this:

• Use Cookie-Free Domains for Components