Resources in the game are finite. You can no longer build infinite units that require a resource. If you have a iron node that gives 2 iron, you can only build 2 units that use iron. If those units are destroyed, the iron is freed back up and you can build other things with the iron, but in essence those 2 units have to be supplied by that node.
The same thing goes with uranium, coal, etc..
There is a civilization which doubles yield from strategic resources, so it's meant as a mechanic.