"Add Channel" Link. The use: You can run multiple channels to keep a log of what is being clicked and/or which website is receiving those clicks.
Doing experiment and analysis is the use, so that you can earn optimized income. BTW, why don't you go and read adsense help. Welcome at DP community.
channel is use to monitor your income per ads. you can set each ads to show in different channel to see which one earn how much per day/click. it's good to use channel to see which ads give you the best income and which one is bad so you can improve that ads for better income. how to use: just create channel by your prefered name (normally name by position of the adsense) then when you create adsense, choose the channel according to the position you will put. then let it run for a while. later, when you check your adsense account you can see which ads earn how much and adjust it for maximum income. ^^
Here's a practical example. Supposing you have two adsense banners, one located in the header of your website, and one located in the sidebar. Supposing you'd like to know which one earns you more money (in case you wish to drop one of them, or if you want to replace them with other ad systems), you can assign each banner a certain channel so they can be identified. This is one simple example. I'm positive that you can find many many more reasons for using channels with AdSense.