Why CTR matters so much?

I'm a newb, so please bare with me: Since you never know how many AdSense ads are CPC ads or CPM ads, is it really that significant to have a high CTR? Could it be that only 10% of the ads are CPC, then CTR wouldn't be so significant. Am I missing something?

CTR means click through rate which means how many people have clicked your ad (on website) or website in (search results)... The more clicks you have the more traffic or money you will get.

This is why CTR is important.

clicks based on your CTR theme