Joke aside, any hoby you'd have, you find plenty of resources. Anything, from programming to cooking, from carpentry to engine repairment, from knitting to forging steel, from chemistry to electronics, from caligraphy to advanced math. ANYTHING. You find tons and tons of resources (on the other hand, _filtering_ through quality material... that's another matter) about any subject.
Then, depend very much how old are you, if you are in your early teens or late teens, you can find chat buddies, watch movies, discover new music (artists AND genres), you can get [yourself hired][1] and so on.
On the other hand, these is viewed by a 30-40 years old me. Most likely, the 15 years old me would just watch those HDs most of the time...
[1]: https://en.wikipedia.org/wiki/On_the_Internet,_nobody_knows_...