String Theory Questions

[WhatMeWorry via Digitalmars-d-learn]

The name string is aliased to immutable(char)[]

Why was immutable chosen? Why not mutable.  Or why not just make 
another alias called

strung where it is aliased to mutable(char)[]

Also, since strings are arrays and arrays are structs with a 
length and ptr
field, I ran the following code for both an empty string and a 
null string.

string emptyStr = "";
writeln("emptyStr.ptr is ", emptyStr.ptr);
writeln("emptyStr.length is ", emptyStr.length);

string nullStr = null;
writeln("nullStr.ptr is ", nullStr.ptr);
writeln("nullStr.length is ", nullStr.length);

and got the following results:

emptyStr.ptr is 42F080
emptyStr.length is 0
nullStr.ptr is null
nullStr.length is 0

I guess I was expecting them to be equivalent.  I can understand 
why both lengths are zero.  But what is emptyStr.ptr doing with 
the 42F080 value? I presume this is a address?  If so, what does 
this address contain and what is it used for?

Or maybe a more succinct question is why not just set 
emptyStr.ptr to null and be done with it?