Switching

Why would I select a port role on say a wifi port or a voice port, at this level.?

This will apply a macro to automatically configure the port to support the endpoint. Here is a explanation and then the explanation plus the configuration that will result depending on what role you select.

https://www.juniper.net/documentation/en_US/junos/topics/reference/general/ex-series-gigabit-interfaces-port-role-configuration.html

http://www.juniper.net/documentation/en_US/junos/information-products/pathway-pages/ex4300/interfaces.pdf

For example if configured for voice and desktop, you want to support buth tagged and untagged  traffic on an access port access with voice vlan. For wifi maybe just untagged/access.