Reason for 'static struct'

[Jon D via Digitalmars-d-learn]

There is a fair bit of range related code in the standard library 
structured like:

     auto MyRange(Range)(Range r)
         if (isInputRange!Range)
      {
         static struct Result
         {
             private Range source;
             // define empty, front, popFront, etc
         }
         return Result(r);
     }

I'm curious about what declaring the Result struct as 'static' 
does, and if there are use cases where it be better to exclude 
the static qualifier.

--Jon