1. as a deliberate effect, either on its own or as part of a reverb; similarly, some synth sounds have this 'built in'

2. as a consequence of multi-microphone recording, where two microphones are used on the same source but are positioned such that the channels end up significantly out of phase

The trouble is that inter-channel phase is one way in which stereo 'works' so it's all a bit of a compromise... this is why there are various tried and trusted spaced stereo microphone techniques that give a sense of spaciousness without introducing too much out of phase content (e.g. ORTF) and often these give a more pleasing result that the coincident-pair (XY) techniques that minimise phase differences. Often in situations where there's lots of sources being recorded at once you can end up with problems. The usual rule of thumb is to ensure that if two microphones are picking up a single source, the more distant one is at least three times the distance of the closer one to the source. A quick way to check this stuff with headphones when positioning microphones is to sum them to mono, reverse the polarity of one of the mics and move them around to minimise the bass cancellation.