First, one has to realize what platforms like YouTube really want. "Inspiring people", "doing good", "educate people" may be side wishes but basically every platform wants one thing: money.
This is not reprehensible - they have to pay wages and infrastructure somehow.
Most of their money comes from advertising. The more time a user spends on a platform, the more ads they are able to show you. But we stay only as long as we are interested in the content on the platform - this is the reason why YouTube has distinct video recommendations. The better they do this, the longer we stay. And the more money they can make.
One problem, however, is the that the content remains permanently online. Since videos take much more time to make the amount of new content is reduced in comparison to Twitter or Instagram. And let us be honest: if we took one saturday a month to watch all the videos we care about, we would get easily through our 'watch-later' playlist and would spend way less time on YouTube. This is not what YouTube likes though. We remember: the longer we are on the platform, the more money they earn.
And that is where Stories come in: if the content disappears one has to be on the platform in this predefined timespan if we do not want to miss it. On YouTube it is 7 days - which, to be honest, surprised me. I can very well imagine that they reduce the timespan to 24 or 48 hours in the future.
But why do Stories work - especially with Fangirls & Fanboys? Well, the stories bring exclusive and time limited content which is very important for fans: maybe the idol is making a Q&A and is answering my question in a future video! Maybe he or she is in the shopping center, only a block away from me and I could get the chance to meet her in person!
You see what I'm getting at? Exactly this fear of missing something 'important' and wanting to be always on the latest leads fans to opening YouTube much more frequently than we would do it elseway. And since we're already in the app we may as well scroll through the home page and watch some videos (and ads).
But that is not the only reason to integrate Stories into YouTube. On the one hand they give content creators another marketing channel for their and brand promotion and YouTube can further reduce the attractivity of other platforms like Snapchat or Instagram. This trend towards monopolization has been apparent for many years - but this is a topic for another article.
YouTube Creator TechAltar has tried to find another reason for stories in general: