I'd rather not look at this as "power differentials" because it implies that one has power over another and in a good friendship and relationship of any kind this is not the case. Power is unimportant in a relationship with mutual trust and respect. True one may be the person who is in control of one thing or another but the other may give that role up willingly and this is ideal. Is it truly a power differential when one is willingly submissive and goes along with things while the other is in the role of organizer, or controls the social events? Not really.

How I see it is where the term Power matters is when it actually, in the mind of the people in the relationship view it as important. Typically this will be where two of the same types of personalities butt heads for the same role. Then it becomes a thing about who get s to be what and can easily be viewed as something revolving around power.

It does exist because almost no relationships are perfect matches where each plays a role that works well with the others.