SpaceX already launches multiple times per month just to maintain Starlink. That will be much cheaper with Starship while enabling larger and more capable satellites. In fact, it's likely that one of the reasons SpaceX built Starlink is to create their own customer (and spur competitors) to plausibly use a significant fraction of Starship's capabilities. None existed at the time.
In the near term the biggest reason to do multiple launches in a day will be orbital refueling. This is required for sending much, much larger payloads to the Moon and Mars. It will require on the order of 10 launches to fuel up one moon lander in orbit, and obviously doing that as quickly as possible will be beneficial. NASA has already committed to this plan for Artemis.
> It will require on the order of 10 launches to fuel up one moon lander in orbit
Require, or just make comfortable? Saturn V had the lift capacity of "only" a couple of Falcon Heavies, but was enough to carry astronauts, a car, a lunar lander with enough fuel to take off, and a command module.
We're not trying to do Apollo again. That would be easier, but we want to build a base this time. For that we need to send a lot more mass and it needs a lot more fuel.
Is that true? SpaceX [0] gives the capacity as 150t reusable or 250t expendable, which is significant, but not enough to make one "enormous".
If you really got order-of-magnitude gains from an expendable upper stage, it wouldn't be that exciting to have reusable rockets (which are more complex and still only fly one order of magnitude more times) in the first place.
If the upper stage wasn't designed to be reusable, it'd probably have a lot less structural dry mass - no heat shield, no fins, possibly different geometry (depending on how much volume they wanted). It might also be possible to drop the sea-level Raptor engines and just use the more efficient vacuum Raptors. I don't think you'd get an order-of-magnitude difference, but it'd be significant, especially for higher-energy trajectories than LEO.
I don't understand why they don't make such an upper stage. It would allow them to refine the booster design and catch logistics while also launching payloads.
Also because of the Lunar Gateway part of the plan. That pretty much just serves to waste fuel and funnel taxpayer money to the companies funding several congressmembers' election campaigns.
The HLS (Spaceship) will need many refuels at orbit in order to get to the Moon and back. That means at least a dozen of fully-loaded Heavy launches to LEO just so each one of them can load a bit of fuel into HLS. The fuel in orbit can't sit idle for too long, or it deteriorates; I haven't found a limit on days for that, but a week-long launch window is considered a dealbreaker, we're talking a dozen Heavy launches in a week.
It's either a short launch window, or at least 6 Starships built and launched twice in ~10 days. Don't count out on SpaceX building 12 Heavy Starships just for that Artemis mission.
200 years ago, there was no need to use electricity. 100 years ago, there was no need to use a programming language. 30 years ago, there was no need for gigabit wireless Internet.
Counterexample: space stations. We've had small manned space stations for decades now, but no real application for them. They're national prestige items only.
And most importantly for anyone who cares about space the way Elon claims to: The ISS has done pretty much all the research we have on how humans survive in space.
We have the means to build pyramids or moa statues (Easter Island-style) much more quickly and efficiently that the ancients did, but we don't feel the need anymore.
If we want to establish long term bases on the Moon or Mars then yes, you need not only to send crew and habitation modules but ongoing supplies and equipment.
Other use cases include launching and maintaining satellite constellations (Starling / Starshield), and launching singular large payloads like space telescopes.
Even for smaller payloads, having both the first and second stages be reusable will reduce launch costs.
In the near term the biggest reason to do multiple launches in a day will be orbital refueling. This is required for sending much, much larger payloads to the Moon and Mars. It will require on the order of 10 launches to fuel up one moon lander in orbit, and obviously doing that as quickly as possible will be beneficial. NASA has already committed to this plan for Artemis.