I believe the point of the converting sentence is that you can convert a $longip() number, which would be base 10 to base 16. If you already have the ipv6 number, then you don't need to do that, but if you're providing the $longip number and don't have the ipv6 number, you probably want to get it into ipv6.
Personally, I don't see any reason why anyone would need $longip for ipv6 either, but that's at least what I think was meant by jaytea and it is a nice feature to include for anyone who really has a need of using $longip with ipv6.
