HackerRank is a collection of programming challenges where developers can come to practice coding skills, prepare for interviews, and get hired. Moreover, HackerRank provides opportunities for you to get certified on different technical skills, giving you an avenue to validate your knowledge and understanding on programming concepts.
Whenever I learn a new language or try to develop some level of competency with a concept, HackerRank has a set of challenges for the topic or technique. So it's a great space to practise that concept by actually applying your knowledge and make you write some code. By being practical in that manner, I can get a better grasp of the idea because I'm building on my experience.
What's more exciting than solving challenging problems? HackerRank has a nice progression system that exposes you to harder challenges as you continue to solve more and more problems. It's a nice way to make programming engaging and focused towards helping you come up with a solution.
The end-goal for many developers is to get hired. I'm no different here. Building on my skills directly helps develop my own knowledge and understanding and consequently improves my chances on landing a job.