Hill repeats can be useful for targeted and sustained strength training, spin development, varied body positioning (standing/sitting), psychological perseverance, etc.

Different terrain will affect the stress placed on your body.
Whether you deem the training necessary or not is a personal decision.

When climbing on the bike, you are in a low-inertia situation. Power application typically begins sooner in the pedal stroke. This is not replicated by low-cadence work on the flats.

Do you need practice climbing to do OK in a hilly race? Of course not. However, if you want to achieve your climbing potential you need to practice going up hill. Only you can answer which end of that spectrum you are on.

With running there is a bit of a difference in that hill intervals appear to help speed on flats. This is primarily from a strength / neuromuscular / musculotendinious improvement that carries over from the high-force application of hill running into fast running on the flats. Unless achilles / calf or foot issues hinder an athlete, they're a staple of most high-performance runner's training diet (at appropriate times of the year).

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