This doesn't mean you will feel the sensor on your skin while you're wearing the watch, but it should allow for more accurate heart rate monitoring. (sammobile.com)
A lot of fitness trackers come bundled with a chest strap, to get the most accurate heart rate readings. (heavy.com)
The more accurate heart rate tracking, the more accurate the measure of calories burned giving you a better picture of how you're doing during your workout. (windowscentral.com)