I think the cans are mostly steel, and may be coated with a thin layer of actual tin to make it resistant to corrosion/rust. While recycling is the least effective of the three Rs, when there's not a real use for the cans (IE, as consequence of not being resealable after being chewed up by a can opener) then returning them to the manufacturing stream via recycling does help reduce the need for mining raw materials from the earth.
You CAN use some cans to make a rocket stove. You could cook a can of beans on a rocket stove made of cans. You can.
Bad humor :D
I don't love using metal containers to start plants. You miss one watering session on a sunny day and your seedlings might all be dead. Or get rust in the soil. Poking holes in the metal will allow uncoated portions of the metal to come in contact with air and water, and cause rust. I thought about maybe using a fancy can opener that leaves a smooth edge to remove both the top and bottom of the cans (it would only work on the cans that have a lip on both the top and bottom) and then hammering the cans flat and using them as shingles. I think it would be very inferior to a roof made of a whole piece of metal, though.