I notice a lot of sites host their resources on a separate domain from the main site, e.g. StackExchange using sstatic.net, Barnes & Noble using imagesbn.com, etc.

I understand that there are benefits to putting your static resources on a separate host, possibly with an efficient static-file web server like nginx, freeing up the main server to focus on serving dynamic content. Similarly, outsourcing to a shared CDN like cloudfront Akamai is logical.

What is the benefit to using a separate domain otherwise, though? Why sstatic.net instead of static.stackexchange.com?

Update: Several answers miss the core question. I understand that there is benefit to splitting between multiple hosts — parallel downloads, slimmer web server, etc. But what is more elusive is why multiple domains. Why sstatic.net rather than static.stackexchange.com as the host for shared resources? So far, only one answer has addressed that.