What people collectively refer to as steeping is actually a few different things:
Leaving the bottle sit with the top off allows alcohol to evaporate. This can mellow out over-flavored juices, or remove the alcohol edge from juices that use a flavor with an alcohol base.
Leaving a bottle sit in a cool, dark area for an extended period of time (anywhere from a few days to a month or more) can allow flavors to blend and allow subtler elements of the flavoring to come out in the
vape. This can make a huge difference for some juices. This is actual "steeping".
Putting the bottle in warm water briefly thins the contents and allows a hurried, partial version of steeping to occur, since the elements of the
juice can more quickly combine and blend in this warmed, thinned state.