that is used for getting the instance of a unit casting the spell... anyway, you can use it for other purposes, I added it so that users can do more...
as you see, EndChannle uses the GetChanneledInstance right? now what if a unit wants to get an instance, no matter if its a channeled or not, that is where GetInstance comes into... for example, if you want to dynamically change the value of a member in another code you can do this
set NovaSystem.GetInstance(unit).npih = 3