The grille on the outside of the cup is the whole design. Instead of trapping the rear wave of the driver in a sealed chamber where it reflects back through the diaphragm, an open cup lets that energy leave. Fewer internal reflections means less smearing in the midrange, and the result most listeners notice first is space: instruments and in-game sources sit apart from one another instead of stacking in the middle of your head.

Everything you give up follows from the same hole. There is no barrier between your ears and the room, so isolation is effectively zero in both directions and deep bass loses the pressure a sealed cup builds. Decide whether your listening space can tolerate that before you compare drivers, and work out how you will handle a microphone, since many of the best-regarded open designs ship without one.