Using a second strobe as a rim light will separate the subject from a background. Separation is especially useful if the background is dark, or while shooting at night.
While taking a picture at night, or in the evening, without a second light the subject might blend in with the background. A rim or kicker light will highlight the edges of the subject, creating a nice separation.
You can see the effect of a rim light in the pictures below:
In
the diagram above, bare strobe at the back is acting as a rim light.