Browsers used to/still do max number of connections per domain. Adding more domains allows you to do more concurrent requests.

Upstreams were very limited, and HTTP requests send cookie data in each GET.

For instance, if you had a 128kbps DSL upstream and each request was 2KB (loaded up with cookies), you're already limited to 8 requests/second. A cookie-less domain for small resources helped this a lot.

Pretty sure in HTTP 1.1, there was a max amount of concurrent requests per server/proxy. So using different domains allowed more rq/s.

A separate domain name for static assets would not receive any cookies of the actual site. This means less data transferred and in theory a proxy closer to the user could more easily cache these requests, even for multiple clients.

More connections = better parallelism = faster downloads

The DNS is cached.

Before HTTP2 this was common

It spreads out the load on disks and outbound bandwidth.