They're going to be used for baked custards!! (:
You may indeed use these cups to make bain marie baked custards. The issues you may have, depending on the type of liquid you pour into them:
1. They are very light, and so - poor conductors of heat. They will float and then tip easily if you have too much water &/or displacement.
2. They can be washed & re-used but will eventually develop tiny holes.
3. Because they are aluminum I would caution against making any custards that are too acidic (caramel, molasses, citrus etc.). As well, you should eat custards within 2 days of baking/cooling, because the longer your custard sits in the cup, the more said custard will pick up a metallic taste.
Many restaurants use these to make desserts that are unfolded to order. I, for one, have unfolded thousands of molten chocolate cakes and pannacottas from them! I hope this helps - have fun :}
Thanks for that fab answer Shuna. I did actually use them for citrus custards and got another great tip from good ol' mum. She suggested to spray/brush the tins with a little oil to prevent the metallic taste when using acidic ingredients. This worked perfectly!