Why do planes fly in the stratosphere the second lowest layer of the atmosphere?

Planes typically fly in the stratosphere, specifically at altitudes of around 30,000 to 40,000 feet, to avoid most weather disturbances and turbulence found in the lower troposphere. The stratosphere has a more stable atmosphere, which enhances fuel efficiency and safety. Additionally, flying at these heights allows aircraft to navigate above commercial air traffic and reduces the likelihood of encountering storms. The thinner air at these altitudes also helps improve engine performance and reduces drag.