In any case, a 6LoWPAN enabled Sensor Network needs a Gateway to interact with an end user who might be using a standard 802.11 WLAN or a 802.3 Ethernet based network.
The Gateway needs to translate the 6LoWPAN enabled IPv6 addresses into addresses that standard networks can understand and communicate. For instance, if a node with 6LoWPAN enabled on it with Header Compression has Address
::1 there needs to be some defined way to know how to access this sensor node over a connected network. For that you need a gateway which does translation for you.
Have a look into 6lbr by CETIC for some more implementation specific example.
Read this Awesome IPv6-WSN-Book for better understanding of 6LoWPAN