Dax,
The bottomline is some people (and I have heard from them) do not like to have to fill in their personal details on a website and choose to go through paypal express to avoid having to do it.
Using it will not only speed up the sales process because the customer does not have to fill in a woocommerce information page (have you done this? any error means you start over) they can purchase without giving you their personal details. Some folks still do not trust online shopping and stick to the Paypal express method