Well the very fact that you are interacting with the system, collapses the wave function. Suppose you want to calculate the air pressure in a Tyre, you draw a little amount of air into your pressure gauge for further analysis. On doing so, you have the reading the tyre pressure prior to your observation. Assuming that the little amount of air you drew for analysis is very insignificant, we can assume the later pressure of the tyre to be approximately equal to the pressure prior to your observation.
On a macroscopic scale, a few units error would not matter. However, if we try to measure sensitive issues that require acute measurements, then, even the tiniest of change can paint a completely different picture.
Since our sensors use photons to observe wavefunctions, the photons interacts with the system and changes the energy of the system.