As it was discussed, monitoring means being able to hear yourself when recording the performance.

Monitoring headphones should not be flat at all. Ones of the best monitoring headphones are 770's and m50x's.

Their spikes are intentional to make the singer record the vocals so that sibilants occur less and for better upper detail retrieval. Narrow soundstage also helps with that.

I don't know if it was designed with that in mind, but we use them for that specific reason as well as many other recording engineers.

And yes, no one is seriously mixing with those.