The difference is one of losing a "forced border". Take, oh, a Victor-class. The reactor is credited with 72MW thermal. The thermal efficiency of a PWR is only about 30% or so, so with the 72MW thermal, it can feed the shaft with about 31000 horsepower (~23MW) and it also has two little OK-2 turbogenerators with a total capacity of 4MW - total of ~27MW of work.
Thus, the Victor's electric systems (to include the coolant pumps, any other pumps, the lighting, the heating, the washing machine, the active sonar ... etc) cannot use more than 4MW.
If you change it with a turboelectric system, then you have about 27MW of electrical supply. Which you can freely apply to propulsion OR any electronic systems (such as a uber-high powered low frequency active sonar) you have. The latter is an important consideration with the increasing proliferation of advanced electronics on ships.
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