FC + CC = TC. If you know two of the values, you can determine the third. You need to know the CC level because if it is greater than 0.5ppm, it may indicate that you have an algae bloom beginning in your water. Or, if you have recently had an algae bloom, it can indicate that you haven't totally killed it off and thus need to continue shocking the pool.

Some pools close with a CYA reading and then open to none. In this case, it means that your CYA has biodegraded and the end result is that you end up with a lot of ammonia as a byproduct. This will show up as a low FC reading and a high CC reading even though your water may look clear. If you only had an FC reading, you would not know that something was amiss..

Hope this helps.