the higher up you go the more sparse the air molecules are the faster you have to go (true airspeed) to collect the same number of air molecules per second (indicated airspeed) as down lower
now even though you're catching fewer air molecules, because your airplane is impacting them so fast the air molecule still can only get out of your way so quickly (mach) so 200kt up high has your aircraft behaving differently than 200kt down low
the interaction between ias and mach meets at the phenomenon known as the coffin corner where the quantity of air molecules are so few that your wings can't keep you afloat, yet the few that you encounter are impacting too fast for your aircraft to deal with, too. this is a major concern for "slow flying" sailplane-like designs like the u-2 near the top of their service ceiling
so for matters of divining your aircraft's aerodynamic capabilities at a given altitude, ias and mach are both relevant, they just describe two different factors that your airfoil cares about -- the quantity of air, and the quality of the air.