Say you really like the tone of your guitar but find it could just be a little brighter and detailed. With the Empress EQ adding a small boost in the range of a of 3kHz to 5kHz with a broad Q can add a bit more detail while still retaining the sound of your instrument. Similarly, adding to the lows would bring out some warmth.
Say you've set your amp up in the venue and find that the acoustics of the stage are making your rig sound too 'boomy' or 'boxy'. By cutting some of the frequencies in the lower mids (200Hz- 600Hz) with a medium Q you can bring minimize the negative effect caused by the room acoustics. Also, if you find your guitar isn't cutting through the rest of the band you can boost in the upper mids instead of just turning up the volume, which could lead to you drowning everyone else out.
With Di'd acoustic instruments playing live can sometimes be a nightmare because of the feedback coming through the monitors. Using the Empress Mid and Hi controls in the 'tight' Q setting you'll be able to cut at the frequencies causing the feedback while retaining much of your instruments tone. With most equalizers the Q is not very tight, so by cutting higher frequencies your instrument gets much duller and 'muddy' sounding.
Having eq control before the amp can shape distortion sounds. For instance, if you want the treble of your signal to distort more you can boost it before the amp. The boost control on the Eq is also great at overdriving the input of tube amps in a very musical way.