The idea is you get a real world bite sized problem to solve.
As I said in the post, my biggest suggestion is to build something on your own, this is where you will grow the most. These challenges are like building something on your own without having to come up with an idea. They are also single problems where many projects are made up of many problems.
Another great thing about these code wars type of sites is you can see how other people solved the problem and frequently learn to be a lot better.
I highly recommend code wars over all the rest. It is really well done and supports a lot of languages. Has built in editor and test runners.
I recommend anyone learning to grow to do at least one challenge a day. Some may only take you 1-2 minutes some may take an hour or more. They are all small problems but sometimes you just don’t know where to start.