A:  Declaring a variable of type : array [0..0] allows you to dynamically allocate the array on the heap and then access the elements of the array with a subscript. Consider the following:

 {$R-} {range checking has to be off or the compiler will complain}
 type
   MyRecordType = record
     Field1 : integer;
     Field2 : word;
   end;
   StructArr = array [0..0] of MyRecordType;
   MyStructPtr = ^StructArr;
 var
   myPtr : MyStructPtr;
   iNumberOfElements : word;
   TheNthElement : integer;
 begin
   {Set the number of elements you want}
   iNumberOfElements := SomeArbitraryValueDeterminedAtRuntime;
   {Allocate the RAM}
   GetMem(myPtr, iNumberOfElements * SizeOf(MyRecordType));
   {Now you have a dynamic array of StructArr[0..iNumberOfElements-1].
    That is, you didn't have to declare the number of elements until
    runtime.}
   try
     {The Nth element is...}
     TheNthElement := myPtr^[TheNthElementIndex].Field1;
     {                       ^^^^^^^^^^^^^^^^^^^        }
     {That array[0..0] declaration makes ^this^ possible. }
     {...and it *must* be a variable.  Constants will cause a range error.}
   finally
     FreeMem(myPtr, SizeOf(MyRecordType) * SomeArbitraryValueDeterminedAtRuntime);
   end;
 end;

Very useful when you want an array, but don't know how many elements you ar going to put in it.