There are some situations where you need to listen to .wav file attachments (like voice mail from an office phone) that are delivered to your email.
I used to listen then through Gmail, but now I have to forward them back to me to be able to listen to them, since I can not listen them while in Basic HTML mode.
He means that there's a lot of advanced programming used in gmail, like xml, ajax, etc, and not all devices can support that stuff. So, they gracefully degrade to a more traditionally coded version for those people.