This gives the dark meat of chicken thighs, which has more fat and connective tissue than breast meat, time to melt into the meat, creating a luscious mouthfeel and deep flavor.
The skin will also crisp nicely, adding a beautiful contrast. For boneless chicken thighs, the cooking time will be closer to 20 minutes, thanks to their streamlined structure.
However, these instructions are merely a guideline. The best way to test for doneness is to use a probe thermometer, which should read at least 175 degrees F.