A surprising amount of people don't maintain a very impressive public profile (me included, tbh). These people need the opportunity to showcase their skills and the only way to do so is by spending some time coding a nice solution.

We try to minimize the candidate's overhead by providing general guidance in the challenges we issue (like "please demonstrate the SOLID principles").

Then we do a code review on the solution, in which we also spend some time. We point out issues, suggest possible improvements and give props on stuff we consider well done. At the end, if the candidate doesn't progress they at least learned something useful. We hope that this way our recruitment process might always be worth the time.