While considering the motion of gas molecules in a container,why do we use rms speed instead of average speed of molecules?

Explanation

Since, a large number of gas molecules is present in a container, therefore for velocity for any molecule there exists another molecule with an equal and opposite velocity As velocity is a vector quantity. Resultant velocity of all the molecules becomes zero. hence average velocity also vanishes. So we could not derive any conclusion about the velocity of the gas molecules from it.
On the other side,if only the magnitudes of velocities (scalar) are considered to find the average , average speed does not vanishes. But in the kinetic theory of gases we find that pressure,temperature , molecular kinetic energy of a gas are proportional to the rms speed are not with the molecular velocity. Hence rms speed of a molecule is more preferable than average speed in kinetic theory.