1. Preventing harm to yourself.
2. Preventing a third party from harming someone else who is unable to defend themselves.
3. Preventing someone from harming themselves (your suicide example).
4. Controlling the actions of someone unable to make their own decisions/choices, in their best interests, as with a small child.
I would also excercise power over another in these cases:
5. To compel someone to act in their own best interests; it must be someone I care about, whose life I am closely involved in. It carries the risk that I actually know what is in their best interests, at least better than they do. It also carries the risk of harming our relationship, but if the situation is dire enough, I will take these risks.
6. To prevent someone from interfering with something I am trying to accomplish, or more rarely, to compel their cooperation; again, to be done sparingly, with full knowledge of the possible consequences.
I am excluding cases where we voluntarily give power over ourselves to another, as in employment, schooling, and voluntary military service.