Yes, creating channels allows you to better track your earnings and also allows advertisers to target the channel, potentially resulting in better earnings..
it is the best way to track the performance of ads on your site. Channels tell you which of your ads are bringing you the most earnings and which of the ads are performing poor....So channel creating is good to track your performance.
Yes, Channels are very necessary, immediately create a channels, if you are not create a channels you will be Ban from Adsense very soon.
No, In other thread you asked "what is the page impression?" I hope you will also ask what is Page eCPM, CTR etc. So it is recommended to you that please read the "AdSense Basics and Policies"