To depict the interactions of objects for a specific task at hand.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
Object Interaction diagrams and Sequence diagrams contain almost exactly the same semantic information. You can convert back and forth between the two diagrams as different views of the same model; Rational Rose can do this automagically.
OIDs are often used earlier in design because they give a nice overview of which objects interact with each other and work well to explore various choices for breaking up responsibility. Sequence diagrams are often considered low-level design artifacts because the time dimension makes them well suited to intricate interactions.
I'll give yet another recommendation for Scott Ambler's AgileModeling.com. It takes some digging but he has a nice set of examples and descriptions of "The Models of UML". Hope that helps!
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi