I think it is important to practice the exercises on those services to improve your logic to solve problems, you never know when is going to benefit you.
Story Time
One time in an interview they made me do one of the coding challenge I did months ago in HackerEarth and they were impressed how well I did it. It wasn't because I am genius or something, I just had the chance to do and practice the challenge before the interview.
So I haven't got a job through these services but by taking the decision to try to do the challenges helped me in that interview. Like I said: You never know when is going to benefit you.