[umanwizard]

But why is that beneficial?

[andreareina]

I'm on a crap connection, and it's not unusual for the styling content to not make it (whether it's CSS or javascript). The ones that follow these principles remain usable, the ones that don't, don't.

[iamben]

Along with what the others have mentioned (connection, screen readers), it's good for your SEO as well.

[shakna]

Bad connections, slow connections and naive screen readers are all common.