Few if any singers get popular performing strictly covers, so while it may not be as fun to hear an original over a classic by Journey, Prince, or Snow Patrol, playing originals may get some of these competitors a head start on their careers directly after The Voice if folks like their songs enough.
The reality is that The Voice did something other singing competitions have struggled with in the past and gave audiences a good grasp of what these competitors sound like when they aren’t performing well-known songs.