This is a terrifically simple and clean way to prepare nice fat fish fillets.
Basically the fish is sealed along with flavouring ingredients and possibly a small amount of veg in a bag and baked quickly so that it steams in its own juices. The bag puffs up nicely during this process, so at least you can tell when the steaming is underway. The main disadvantage being that there's no way to rescue an undercooked bag, so fish is the ideal content - it being fairly difficult to disasterously undercook fish.
There seems to be no functional reason to prefer parchment or greaseproof paper parcels over tin foil, except that the crispy paper puff looks nicer if you serve it at table, and it avoids accidentally eating any bits of foil.
However, paper is harder to seal and has a tendency to burn, so you see chefs faffing about smearing their parchment with oil to protect it, and brushing beaten egg whites along the folded edges to seal the parcels like pastry.
I rather like Rick Stein's take on this which is to use a sheet of tin foil on the fishy side and cover it with parchment on the outside. That way you get the restaurant look of golden parchment with all the foolproof convenience of tin foil.
Bonus!

It was Sunday when I cooked this, so obviously no fishmonger was open. Why would they open so that people who earn money by working during the week can buy fish at the weekends when they need to eat? Ludicrous.
So I bought my fish at a Fucking Supermarket™ which meant the only fish available were some thin, bony skinless fillets of Haddock. If you can, find yourself nice fat fillets of some solid white fish, preferably with the skin still on. I would have bought Hake if there'd been any.