This is a valuable tool for estimating TSS using Rating of Perceived Exertion or RPE. Sometimes you don’t have a power meter and heart rate strap when doing a ride, run or swim. Using this method, you can still estimate your RPE.
For example, on my Mohican ride yesterday, I rode my mountain bike, which doesn’t have a power meter and I didn’t have a heart rate strap. I can still come up with a TSS number based on time and a RPE of 6. The calculation is 3.5 hours * 70 RPE per hour (3.5 * 70 = 245).