The YouTube Partner Program (YPP)was created to allow creators (YouTubers) to monetize their videos on YouTube. Creators earn money from advertisements served on their videos and from YouTube premium subscribers watching their videos.
A YouTube channel must have at least 4,000 watch hours in the previous 12 months and at least 1,000 subscribers before it can be reviewed to join the program.
There were three founders in partnership who worked for Paypal - Chad Hurley, Steve Chen, and Jawed Karim. They started a dating site inviting women to upload videos of themselves. None did. Karim filmed himself at the zoo and then people used youtube to upload things that interested them, like their cats or their holidays.
So the three let it happen, made it more open and decided to let the public upload videos of what they themselves wanted to see. It worked.