Why is recycling cans and tins a good thing to do -?

It costs money and energy to extract the raw materials for things like tin cans from the earth. Recycling old cans into new ones (a) means you don't have to mine the raw materials, and (b) uses less energy to melt and re-corm the cans. The raw materials will not be around forever - the more we recycle - the less we have to rely on the earth to provide for us.