SpaceX already has high bay and Megabay. Why do they need another Megabay? They don't seem to be using the existing megabay at anything like capacity. Possibilities:
*need more build capacity
*need to upgrade for taller boosters
*need to handle heavier loads
*want to bring all construction inside, away from weather and prying eyes.
If the existing Megabay is obsolete, they might use to to store SH and SS, or they might upgrade it. If they intend to increase capacity, the how will they move the product that those increased volumes?
When they begin re-use, there will (I suggest) be extensive examination and likely some repairs, and minor upgrades. That will take Megabay space. (Like F9 there will be a fleet refurbishment and storage process.) This could be needed as early as mid 2024!
The rocket garden is pretty full. Perhaps they will use it for storage as well as fabrication.
Or maybe they are looking to the future. Hoping that future tested go well and to be prepared to ramp up production.
When they begin re-use, there will (I suggest) be extensive examination and likely some repairs, and minor upgrades. That will take Megabay space. (Like F9 there will be a fleet refurbishment and storage process.) This could be needed as early as mid 2024!
But this implies that launches will primarily be at BC. As of now we have no evidence that they will launch more that 5/yr from BC. They don't need more than about three boosters per site, so If it's mostly for refurbishment we would expect them to build shorter building.
i reckon because they would want to improve the production capacity at boca so they can have boosters that go to florida before roberts rd is operational.