It's not really anything to do with "an era where these [...] provided a pretty steady level of energy", it's just the only way that makes sense to describe it in any case.

Demand is always instantaneous, and transmission lines from a generator need to be sized for the maximum instantaneous generation, so in terms of the size of (any kind of) generator it's the main thing you're interested in. Capacity factor brings in seasonal / daily output variation but that's a whole different thing.