Originally Posted by studiocritic

in summary, google wants to see an ad that is going to earn them the most money.

so that means it has to be relevant and well written. thats what raises CTR.

google does not want someone willing to pay $4 with a 0.1% CTR. they'd much rather have someone with a 40 cent bid, but a 15% CTR. The latter ad, although only bidding 10% of the other, will yield them 150x more earnings.