According to a Teslerati article linked from another QA:
The most critical component for the booster, according to Musk, is that there is “a lot of force pushing up.” He states that to launch Starship, the Super Heavy booster would ideally produce roughly 7500 tons of thrust – about twice the thrust of that of a Saturn V rocket. For a reusable rocket design – such as with the Super Heavy booster – a high thrust to weight ratio (about 1.5:1) is a necessity for efficient operation.
Most modern launchers have an initial thrust-to-weight ratio off the pad of about 1.2:1, with some outliers.
The excess above 1:1 TWR determines how long it takes to clear the launch tower, so less than around 1.2 isn't desirable. Above that minimum TWR, my understanding has been that carrying more fuel in the first stage is generally better than not; the dry mass penalty of slightly larger tanks is outweighed by the acceleration potential from the additional carried fuel.
What is it about reusable rockets that makes a higher TWR "a necessity for efficient operation"?