Someone asked this question over at the Home Recording Forum:
If you're getting the right tone from the start when you record, for example using the tone control on the guitar and its amplifier, why is EQ needed in mixing? I've read a bunch of articles singing the praises of EQ, but in the mixes I made I didn't touch it and they worked fine. I think I only used it once, to round off some top end on a guitar that was annoyingly bright.
Why is it considered so important?
I answered the question with the following:
You're right about EQ's necessity if you get the tones right at the source. Ideally it should not be needed, especially if you only have one or two tracks to deal with (say a voice over with some royalty-free music). Many folks over-use EQ to "fix" bad recording techniques. HOWEVER, in music recording, when you have multiple instruments playing together on multiple tracks, things can get muddied up pretty fast. Using EQ to assist in the mixing process becomes very important. The best mixes make room for every element to be heard, and without some EQ, certain instruments tend to mask each other by competing for the same frequencies. One common example is bass guitar and kick drum. They share many of the same frequencies and the best way to let them both be heard more clearly, you have to de-conflict key frequencies, reducing bass in one frequency area (letting the kick through), and reducing the kick in a different area (letting the bass through).
Hopefully that made sense. Basically, the more elements you have in a mix, the more likely you are to need EQ on individual tracks. But if you're just doing, say, a voice over track, you shouldn't need any EQ at all if you have a good recording space and decent signal chain.