Imagine a scenario where half of your ads are loaded below the fold and nobody is scrolling down the page. Even though the ads are not displayed in front of the visitor's eyes, impressions are counted because the ads were loaded. Since CTR equals impressions / clicks, your CTR can never reach higher than 50% here.
When lazy-loading is enabled, an impression is logged when the ad loads in front of the visitor's eye. Logically, when the ad is seen, it has a higher chance of receiving a click. Therefore it drives up CTR.
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 container holding the ad. 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.