why internal and external

This is for very basic configurations, and is rarely used in my experience. If you know enough about your network I typically recommend to click the "Finished" button on the initial setup, and to configure your interfaces, VLANs, etc. manually. The basic configuration assumes you will be routing through the BIG-IP, hence the Internal and External names. Often the BIG-IP may be configured in a one-arm mode, and then use SNAT. If you are using a routed mode configuration it would be more likely that you would want more than 2 VLANS, placing a VLAN and SelfIP on each network that will house servers, and a "transport VLAN" on the "External" side that will have the inbound routes for the VIPS, and the default route out.