A good EQ pedal is a wonderful tool to get that 'nth' degree of tone from time to time. For me, it's a mandatory add to any pedal board. I often use one with my amps to accentuate or cut certain frequency bands with different guitars - for example, to add a little edge, definition or glassiness to humbuckers, or add a little fullness to single coils. Or cut excessive highs or low mids.
An amp's tone controls often have turnover points at similar frequencies, but their 'Q' might be too wide or narrow to do the job, and sometimes it's just not the frequency you need to alter in the first place.