? toString(?)

[simendsjo]

Object contains toString as the following:
string toString();

But there are also other versions going about:
void toString(void delegate(const(char)[]) sink, string fmt);
void toString(void delegate(const(char)[]) sink, FormatSpec fmt);

I wouldn't be surprised to see some range based version too.
Where are the different versions required, and what is best practice 
when implementing toString?