As far as I know,
WAN miniport drivers allow modems to connect to the internet through your PC using a variety of protocols.

WAN stands for Wide Area Network.

A little more information form MS:

WAN miniport driver uses links to send and receive data. Links are logical, point-to-point bidirectional communication channels. There can be many links per NIC. Links are dynamically established and torn down. The link speed and quality of the link can vary for each connection.

So it allows your NIC to send and receive data over the net. However, if your modem doesn't require it, then, you don't necessarily need it.

On many modems use different methods:

WAN Miniports and RAS Components
WAN Miniports and the RAS Async Adaptor components have indirect dependencies on Internet Explorer. Unless WAN miniports are required, you can remove the following components to avoid adding in Internet Explorer and all of its dependencies:

RAS Async Adapter
WAN Miniport (L2TP)
WAN Miniport (IP)
WAN Miniport (PPPOE)
WAN Miniport (PPTP)

Hope that helps,
BB