hacker.org

or are these challenges just for fun?

I don't want to waste my time and realize I'm still a noobie hacker once I spend a year doing all these challenges.

Many of the challenges require you to write some code, so you can practice your coding skills that way.

it is a REALLY good way to practice your hacking skills and even to earn new skills
i must admit that now I am more skilled in solving problems thanks to these challenges