Two reasons: information and freedom. People go online to get information (music, news, stock prices, etc). Logging onto the Internet is like a kid going to a toy store; there is so much to do and explore that once you get in, you never want to leave. People also go online because the Internet offers freedom to do whatever they want, to say whatever they want and to be whoever they want. This could have a negative effect since people will act in certain ways and say certain things that they otherwise wouldn't do in real life. Furthermore, because the Internet offers so much freedom, you can find your ideal self much easier, but it's not the same as your social self. The difference is due to you talking to a computer screen and you talking to a real person. Thus, the experiences we encounter on the Internet are not the same as the ones we encounter in real life.
Belinda