When testing a battery, you need to put approximately the starter draw amperage on the battery for an extended period of time. This tests the reserve cranking capacity of the battery. Normally a 15 second load time is used. A 15 second continued cranking on a started is not at all good for the starter. A carbon pile load tester can simulate this load without damaging the starter.
In order to properly test a battery, you have to put a heavy load on the battery for a given period of time to see how much cranking power the battery actually has. As batteries get older they can still hold a bit over 12 volts but as soon as a load is put on them the voltage drops below what it takes to start the engine.