The top ad unit is the very first thing that users see when they open YouTube, so avoiding advertising for drugs, alcohol or gambling will help improve the perception of the page content and make YouTube acceptable for different social and age groups of users.
In terms of politics, this can help the platform avoid controversy, for example, during campaigns and candidate debates. And the ban on alcohol advertising will make the service more suitable for underage users.
Please note, Google Ads will launch experiments for YouTube video ads.
A source: The Verge