Every time someone does a search that triggers an ad that competes in an auction, we calculate an Ad Rank. This calculation incorporates your bid, auction-time measurements of expected CTR, ad relevance, landing page experience, and other factors. To determine the auction-time quality components, we look at a number of different factors. By improving the following factors you can help improve the quality components of your Ad Rank:
Your ad's expected CTR: This is based in part on your ad's historical clicks and impressions (excluding factors such as ad position, extensions, and other formats that may have affected the visibility of an ad that someone previously clicked)
Your display URL's past CTR: The historical clicks and impressions your display URL has received
The quality of your landing page: How relevant, transparent, and easy-to-navigate your page is
Your ad/search relevance: How relevant your ad text is to what a person searches for
Geographic performance: How successful your account has been in the regions you're targeting
Your targeted devices: How well your ads have been performing on different types of devices, like desktops/laptops, mobile devices, and tablets
How ad quality affects you
The quality components of Ad Rank are used in several different ways and can affect the following things in your account:
Ad auction eligibility: Having better quality components typically makes it easier and cheaper for your ads to enter an auction. Our measures of ad quality also help determine whether your ad is qualified to appear at all.
Your actual cost-per-click (CPC): Higher quality ads can often lead to lower CPCs. That means you pay less per click when your ads are higher quality.
Your keyword's first page bid estimate: Higher quality ads are typically associated with lower first page bid estimates. That means it's easier for your ads to show on the first page of search results when they have high quality components (expected CTR, ad relevance, and landing page experience).
Your keyword's top of page bid estimate: Higher quality ads are typically associated with lower top of page bid estimates. That means it's easier for your ad to show towards the top of the page when your ads are higher quality.
Ad position: Higher quality ads lead to higher ad positions, meaning they can show up higher on the page.
Eligibility for ad extensions and other ad formats: Some ad formats require a minimum ad quality threshold to show. In addition, your Ad Rank determines whether or not your ad is eligible to be displayed with ad extensions and other ad formats, such as sitelinks. Because Ad Rank is a function of the components of Quality Score, higher quality ads can increase the likelihood that your ad is displayed with extensions and other formats.
In a nutshell, higher quality ads typically lead to lower costs and better ad positions. The AdWords system works best for everybody – advertisers, customers, publishers, and Google – when the ads we show are relevant, closely matching what customers are looking for. Relevant ads tend to earn more clicks, appear in a higher position, and bring you the most success.
https://support.google.com/adwords/answer/2454010?hl=en