The vast majority of speakers have an on-axis response that is different from their off-axis responses. The disparity in frequency responses is something our brains learn to recognize as "loudspeaker sound", which harms the illusion. If you EQ one direction flat (usually the on-axis aimed for the sweet spot), the off-axis responses can get even worse. You can't fix directivity problems with one dimension of EQ.
OTOH, is EQ popular for headphone listening?
The non-directional bass region is a good place for EQ, but most *graphic* equalizers don't have the needed range, precision, or gain for that job. *Parametric* EQ is best, and shelving filters are helpful. This is where DSP shines, with presets, time delays, automatic correction, and accurate displays of calculated total results.