Equalisation, or just eq technique allows you to gain or diminish certain frequencies in the way you want. It is a crucial process in the modern music industry whether you are doing acoustic or non-electronic music pieces. The great use of eq helps people cut off the undesirable sound from instruments and audio and bring out the frequencies that are wanted.
Other than that, do you know that audio equalization was not originally developed for music production? It was developed to regain the high-frequency loss during the long-distance signal transmission of early telephone communication. As far as we know the idea of filtering audio frequencies came out at least before 70s of 19 century. 1
1, Välimäki V, Reiss J D. All about audio equalization: Solutions and frontiers[J]. Applied Sciences, 2016, 6(5): 129.