I battle with the same thing, but I can really relate to Chris - her reasons and purposes.
I need someone objective to vent to, who can at the same time be unbiased enough to assess if I'm coping. I like to know I have someone I can phone when I feel I'm not coping, who can make the decision around - do I need to be admitted to hospital, should I be phoning my pdoc to change my meds...?
I just like knowing there's a point of reference out there.
She can help me analyse my emotions
She can be a support when the bottom falls out.
I find that my "friends" never really understand. They do not suffer from bipolar, they cannot understand the pain and anguish. They'll probably compare the moods to PMS, depression to "The Blues". I need someone in my life who understands.
I want to learn coping skills to get through the depression on my own (Unless I'm really at rock bottom)

Those are just my 2 cents