It is a very honest and good question!!!
We have IAS
We would like to have EAS, which happens not to be a speed, in the velocity sense.
But there are errors. CAS is IAS corrected for position and instrument errors.
If we corrected also the compressibility error (which stems from the pitot tube compressing the air, and this compression is sensed as increased speed) then we would have EAS.
We would like to have TAS, too, because we need it for navigation and other stuff. TAS is the speed relative to the air mass in which we are flying. This one is truly a velocity. If we add the wind effect we will have GS, which we obviously need for navigation purposes. And of course it is a velocity.
EAS, and its children (CAS and IAS) is a way to read pressure in knots. At sea level in the ISA atmosphere, EAS and TAS are the same. EAS is the speed that you
would need in the ISA at sea level to get the same dynamic pressure as you get at your TAS in your actual air mass. It is not an actual velocity. You need to know it because dynamic pressure is what makes the airplane fly.
The relation between TAS and EAS was arbitrarily stablished by some scientific. It is a convenience. There is no such thing as the "density error". Yes, you will read about it but we need EAS. TAS is not good for flying purposes.
All in all, the changes are very small mostly because the stall speed is a relatively small number (the smallest in the normal flight envelope). If an airplane stalls at 120 KEAS at SL, it stalls at 120 KCAS. If it's at 35,000' the KCAS becomes 121.6 KCAS - a difference of only 1.6 knots!
CAS is a very erroneous speed at 35,000 ft. The difference between IAS and CAS can be small, but the difference between EAS and IAS is very big, in the range of 15 kt. That is why stall speed increases with altitude. equivalent stall speed remains relatively constant, but IAS and CAS will increase with mach number.
Also, compressibility effects also have a part in stalling and even stall EAS will vary with mach number.