When you want to say exactly what you mean; IM is the answer.
I've had some of the most intimate and intense conversation of my dating life over IM. Sitting at my desk (at my old job!) I would hash out the boundaries of my relationship, try to keep from crying and remain alert, ready to minimize if a co-worker walked behind me.
Over at Nerve (full disclosure: that's where I used to work) Will Doig writes about the non-stop IMathons that happen between sweeties when both of you are at work. Will and his boyfriend IMed so much, "it got so that IMing began to feel as natural as face-to-face conversation, and our online relationship even developed its own tone, a goofy sort of dialect that, if you're an IM user, you might be familiar with."
Will and his ex also broke up over IM, which brings me to the point of this post: IM can be a great way to have serious conversations. Without the face-to-face pressure you're free to figure out exactly what you want to say, exactly which words will express your nuanced feelings. When you're angry and want to spit out the first mean phrase that you think of, IM provides a valuable break peddle; in the split second before pressing send you can decide if you really want to tell him that you've always hated the way he kissed.
Of course, IMing at work has it's own risks: you can get called away for a conference call in the middle of an important conversation, and you can't fully immerse yourself in whatever overwhelming emotion you're feeling. But maybe even that's good; retaining a sliver of objectivity during excruciating conversations can put things in perspective. (The other downside: lost work time.)
Will ends the piece by describing how, in his new, IM-free relationships, he enjoys holding back useless bits of information and trivial office gossip so at night he and his beau can talk about everything at once; "had we dished them out piecemeal, they just wouldn't have tasted the same." I'd add that while important conversations are usually also better in person, the IM relationship discussion is not to be underrated.