Jam roly poly - why the tray of water?

[marmitedoughnut]

Fancy making a jam roly poly so have been looking at recipes online - pretty much all of them state to wrap the pudding in parchment paper and then foil and to cook in the oven above a tray of hot water.

What's the point of this? surely if it's wrapped in foil (and sealed) the steam from the water won't do anything.

[hugoagogo]

That's how you steam it, just like Christmas pudding.
The steam doesn't need to get into the foil, it just cooks the pud more gently than just baking.