Each affiliate allows you to run detailed reports. This is helpful when you start an ad campaign for one of your hubs. Say you have two ads and you want to see which is most effective to drive traffic to your hubs. Run one ad for a few days than another for a few days. Both AdSense and Amazon will let you run reports based on the criteria you choose. You will quickly see which ad had the most clicks and highest CTR. This can help you to write better ads which will translate into more traffic to your hubs.
Another thing that is cool about these reports, is they allow you to track earnings by day, week month or a custom time period of your choice. This has helped me set personal goals to see if I can increase my AdSense earnings every month. So far I have been able to do so, kind of a game I play against myself. Except in this case the higher the score equates to more $$$ every month!!!