Studio monitors, also called reference monitors, are loudspeakers specifically designed for audio production applications such as recording, film, television and radio studios where accurate audio reproduction is crucial
When the term 'monitor speaker' is used among audio engineers, there is likely to be an assumption that the speaker will be designed to produce relatively flat (linear) phase and frequency responses. In other words, there will be no emphasis or de-emphasis of particular frequencies so that the loudspeaker gives an accurate reproduction of the tonal qualities of the source audio ("uncolored" or "transparent" are synonyms), and there will be no relative phase shift of particular frequencies meaning no distortion in sound stage perspective for stereo recordings.
other words they need to be accurate in the first place...