Probably the least exploited kind of effect pedal because it's so contrary from what we think of in a pedal. Sometimes good tone comes from a subtle boost in one spectrum or a subtle cut to accentuate and cut through the mix. At other times, creative EQ'ing at the end of your chain can shape a sound that wasn't there. Isn't a Wah pedal just a variable EQ pedal?
EQ'ing is where the men and boys are separated. Underestimate an EQ pedal at your own peril.