EQ's are very useful. They can be used as boosts for solos or maybe a hard-rock chorus. They can be used to take the high end of something like a Tele or add lots of bottom to something thin like a Strat or Tele. EQ's on amps don't make huge differences. On an amp, say on the Treble setting, most people will not notice a difference between 2 and 3. However on an EQ pedal, such as MXR or BOSS, a slight change in the slider will make a noticeable difference.