BeN2 wrote:
I don't understand the purpose of this practice.
You are only considering the risk of overrunning the scheduled landing distance. The other risk is that of arriving over the runway with insufficient airspeed to conduct a proper flare. That situation may arrive due to an adverse gust, or due to the natural reduction of windspeed with height above ground. The reported windspeed (assumed in the calculation of landing distance data) is measured at (or corrected to) a height of 10 metres above the surface. The actual steady windspeed typically varies as the 1/7th power of height above the surface. The calculation of required landing distance data takes into account 50% of reported headwind component, and 150% of reported tailwind component.