This is a simple question, in essence. But the deeper you delve, the more interesting things sometimes are…
Firstly, EQ stands for 'equalization'. But that implies making one thing equal to another. So what are we making equal to what?
The answer lies in history – the history of communications technology rather than sound engineering. Back in the early years of the twentieth century, scientists and technologists were trying to send voice communications over further and further distances through cables.
But the longer the cable, the more mangled the signal became. Repeater amplifiers could boost up the level, but some frequencies – high pitches, medium or low pitches – got through more easily than others.
So equalizers were employed to bring back the relative balance of the frequencies to what they would have been originally. Equalizers were also used in other areas of audio, including broadcasting.
In the late 1950s and early 1960s however, recording engineers and producers started to realize that an equalizer could just as effectively make things unequal, this time for artistic reasons rather than technical.
So the devices that were once employed as a corrective measure now became creative devices. And they developed from there.
So the equalizers we now use are employed for anything but equalization in the true sense of the word. They still have their corrective uses, but generally they are a creative tool in the engineer's armory.