Even though there is a clear progression in price and primary parameter while going up the line of E->A ratings, every rating has its niche.
Internals with different ratings are balanced(design compromises preferring different parameters) differently:
Taking the example of a power plant (but also valid for the others):
E - cheapest, but bad at everything
D- Lightest
C - ????
B - Armoured but also the heaviest
A - Most powerful, also best heat management
So far i heavent been able to determine the the role of the C rated components, or even if they have any.
It would be interesting to make the C rated ones to have the best heat management (instead of A).
Like others have already said
C = decent performance in the medium weight class E,C.A and very useful if you run short on power to run something better, or you can't afford the A yet.
It marks the center point of modules and its still a rather good deal for the price.