Mines produce Iron, which is sometimes called "Ore" by people. If you run a lot of boss rallies you will probably want to max out the number of Mines you have in your city in order to negate the cost of constantly healing your Cavs.
Iron is used in all building projects and is used in the training of every troop type minus Archers.
For real though. Don't open your resource boxes until you need them. Even if you don't think you're bubble will drop you should treat it like it can go down at any time.