What I don't see mentioned: at the end of the day, Starship is designed to be fully and rapidly reusable. The core / the most important part of that goal is obv

What I don't see mentioned: at the end of the day, Starship is designed to be fully and rapidly reusable. The core / the most important part of that goal is obviously to recover the 1st stage (not throw it away) and to recover it safely and with minimum delays. That's why, once operational, almost all starships will have their booster return to launch site. To return to launch site, you need to keep some of the fuel in the booster to flip it, do a boostback burn and a landing burn ( starship boosters do not need entry burns but they will need landing burns that are not suicide burns ending with hoverslams like falcon does, but basically end almost at zero velocity, hivering as the tower arms close around it). The primary reason for hot staging is that without it, they had to turn off all engines, mechanically "push" the ship away from the booster and then light the ship engines. Say this takes 5 or 10 seconds. During this time, the entire system is not accelerating anymore, but it's still screaming away from the launch site (because to go to orbit, you don't just go "up", you mostly go "sideways", that's why you often hear "vehicle pitching down range" during broadcasts) and to get the booster back to the launch site you then have to spend more fuel to get back to the launch site - you have to counteract all that time you spend moving away from the launch site. With hot staging you get to the velocity you need to get the first stage to orbit sooner, because the system never stops "pushing", and there's less distance you need to cover to get the first stage back to the launch site. With means you need less fuel that remains in the first stage after stage separation, which means you can use more fuel during the first stage firing (and therefore put more mass to orbit).