The mixture is easier to ionize than either neon or argon alone, and lowers the breakdown voltage at which the tube becomes conductive and starts producing light.
The argon changes the color of the "neon light", making it slightly more yellowish.
The neon gas used in some nixie tubes includes a small amount of mercury vapor (for various reasons), which glows blue.
The Penning mixture used in plasma displays is usually helium or neon with small percentage of xenon, at several hundred torr.
Penning mixtures with the formulas of argon–xenon, neon–argon, argon–acetylene, and xenon–TMA are used as filler gases in gaseous ionization detectors.