A miniport driver (in the LAN / WAN context) is a piece of code concerned with the details of the device (such as a NIC (network inteface card)). It is not concerned with the wider aspects of networking. This is handled by NDIS (network driver interface specification) which apart from being a spec. is real code provided by Microsoft.
This makes the device driver easier to write as all the standard stuff (the "wrapper")is provided. If NDIS did not exist then manufacturers would have to write and test shedload more code.