James left Cloak with DD - Why?

[Hagrid]

No: HPFGUIDX 155487

> "Hagrid" <aussie> wrote:
> >
> > Christmas in PS/SS, 
> > 
> > "Your father left this in my possession before he died. <SNIP>
> > 
> > So why did James leave it with DD? Not for DD to use ...
> > 
> > "I don't need a cloak to become invisible," DD 
> > 
> > Any ideas? Someone escaping (RAB), or someone spying?
> >
> 
> Tonks:
> I have always assumed that it was left behind when he was a 
student. 
> Sort of like the Mauraders Map was left at Hogwarts. I have never 
> thought that there was anything important about it. Maybe it was 
> conficated for some wrong doing and James just never went back to 
> get it.
>
aussie:
I know. I skimmed over it a few times without thinking. But Ron had 
said, "they're really rare, and really valuable".

The Map is useful only at Hogwarts, but we have seen other things 
the cloak is useful for since then.

RARE? - Snape has to use Harry's cloak in POA
- I think we have only seen the Crouch's with a Cloak, no-one else.
- OOTP get Harry to use it to leave Dursleys - but others didn't 
have one
- DD tells Harry to keep it with him all year in HBP.

So why did a person in hiding not need something that makes him 
invisible? ... What greater purpose did DD have for it?