Because as far as I know Mach depends on altitude, perhaps you mean the speed of sound (which depends only on temperature)?
Mach is a measure of percentage of speed of sound, somewhat speaking; it's used in aircraft as a measure of compressibility in terms of speed.
Generally speaking, airspeed is useful at lower velocities when compressibility isn't a factor. Subsonic air, particularly at lower speeds, is considered incompressible for calculation purposes. As we approach the transonic region (roughly .75 mach on up to about 1.2 mach), we encounter compressibility effects, and it is approaching these speeds that we begin to consider mach. Below that, we consider speed as a function of airspeed, rather than mach.
This isn't a hard and fast rule, of course. Some aircraft are limited by mach speeds, especially at higher altitudes, at mach values lower than the traditional transonic range. These aircraft also reference mach, but only when mach becomes the limiting factor (during a climb above about 27,000' during normal operations), rather than airspeed.
At higher altitudes and lower temperatures, airspeed values continue to drop as we climb, with a corresponding constant value mach. Mach becomes of interest primarily as a cruise value and a limiting value. We generally calculate our minimum and maximum airspeeds during high altitude cruise as well, to determine "buffet margins," which tell us about high-speed buffet (onset of mach effects--tuck, buzz, etc) and low speed buffet (traditional aerodynamic stall and associated effects).