Why YouTube is so popular?
Same as Facebook, The range of audiences is so large, everyone can relate to it. Also I think more of the population, especially these days are drawn more to watching video then anything else
What purposes does YouTube have?
YouTube is used for everything not only moving image but pictures, texts and even music. It is used for educational purposes, (Tutorials, lessons and research,) entertainment, (Music and film.) also YouTube can be used as a vlog and watch things live.
Many celebrities today started by uploading their videos to YouTube.
Gabrielle Aplin who is now a famous british musician, sold herself by uploading her own songs and covers to her YouTube Channel.
Esmee Denters build her career by uploading low-tech karaoke videos
Felix Arvid Ulf Kjellberg Also known as Pewdiepie is now one of the most famous people on Youtube, just by sharing his gaming experiences and other vids. PewDiePie earned $7.4m 2014 due to his large amount of views and subscribers.
All these people have got where they are now due to turning to this website to sell themselves.
Contextual Awareness in YouTube
Like most websites, they use ads as a way of trying to persuade you to buy their products. Most of these ads are linked by your previous searches. If you like a game page on Facebook or watch videos of games on YouTube, YouTube will record this and advertise related videos. They do this by asking you to view or subscribe related videos.