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.

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.