You are basically allowing an ejuice to sit for anywhere from a few hours to a
month or more. Steeping allows things to "settle in" so-to-speak. Many juices
can benefit from steeping, and their tastes will change over time. Some juices
that you may have thought tasted horrible when you first get them or create
them, may be the best thing ever after sitting for a few days or longer.
Never toss a
juice you don't like without letting it steep at least a few days
and try it again. You never know, you may end up loving it in the end.
Hope this helps,
Patrick
Also see:
Steeping - Wikipedia, the free encyclopedia
...for more technical detail on steeping.