How does strings’ immutability help in string interning?

If String were a mutable type, it simply couldn't be interned. There's no point in keeping a pool of objects if they can be mutated... you wouldn't be able to rely on what you got out of the pool being the string you were interested in (at least by the time you used it).

Imagine:

string x = "foo";
string y = "foo";
// x and y are references to the same object...

x.MutateTo("bar"); // This doesn't change which object y refers to...
Console.WriteLine(y); // This would have to print "bar" - eek!