The advantage to a partly electric airplane is similar to the advantages of a hybrid car where a small motor charges a big battery: You can size the engine for cruise, and use the battery as a temporary booster for takeoff, goaround, or failure scenarios.
An engine sized for cruise will be more fuel efficient than one sized for full blast takeoff power.
Whether that incremental efficiency from cruise-sizing will pay for all the rest of the e-system’s size, weight and complexity is the question. In the case of cars the answer is “definitely yes”. In the case of smallish planes the answer today is “yes, but it’s not a slam-dunk total game changer. Yet.”