For platforms, it’s all about attribution and proving ROI (/ROAS) in a world of increasing privacy.

For Google (/YouTube), enter Engaged-view conversions:

Engaged-view conversions are counted when a user watches at least 10 seconds of a skippable in-stream ad (or watches the entire skippable in-stream ad, if it’s shorter than 10 seconds) and then converts within the Engaged-view conversion window.

Google: About Engaged-view conversions