Why hashtags, keywords matter in youtube & Facebook videos?

I know that when we upload videos on a Facebook page, there is a section where we can set the keywords or tags. Is there any use for this? Could anyone please tell me what tags are important in Youtube and Facebook video uploads? How they impact marketing?

Hashtags can improve your videos SEO performance and also they help you to reach more audience and gain more views for your videos that can improve your brand visibility.