Lazy Loading has two major advantages when it comes to ad serving:
Imagine a scenario where half of the ads are loaded below the fold, and nobody is scrolling down the page. Even though the ads were not seen, impressions are counted because the creatives were loaded. Since CTR equals impressions / clicks, your CTR is also cut in half.
Clearly, only ads that have been seen can be clicked on. When lazy-loading is enabled, an impression is logged only when the ad is displayed in front of the visitor's eye. By reducing the number of impressions of unseen ads, your CTR will increase considerably.
A viewable ad impression is determined as 50% visible for at least 1 second. If you enable lazy-loading, the ads are only loaded when the visitor scrolls down to the ad unit's placeholder. At that moment, the ad is loaded and the clock starts ticking in order for a viewable impression to be counted. In other words, enabling lazy-loading increases the chance that an impression is logged as viewable, and as a consequence, your viewability rate goes up.
Finally, pages load faster when lazy-loading is enabled. Especially on web pages with multiple ads, it's strongly recommended to enable lazy-loading in combination with asynchronous ad loading. If you're a publisher, Google may even reward you (i.e. not punish you) when page speed improves.
Here's a live example of an ad that is only loaded when you scroll it into view.
Go to the page Zones and open a Display Ad zone. Click on the Advanced tab and enable Load When In Viewport.