Some merchant sites may not know the total or line items at the time of attempting to open the payment sheet with show(). This feature is a way to signal that the browser should show a spinner or equivalent and wait on allowing end user interaction until an update.
Samples: https://rsolomakhin.github.io/pr/wait/ https://rsolomakhin.github.io/pr/mod-wait/ https://rsolomakhin.github.io/pr/no-wait/ https://rsolomakhin.github.io/pr/single-wait/ https://rsolomakhin.github.io/pr/multi-wait/ https://rsolomakhin.github.io/pr/us-wait/ https://rsolomakhin.github.io/pr/wait-skip-ui/ https://rsolomakhin.github.io/pr/ko/reject-show-promise/ https://rsolomakhin.github.io/pr/ko/timeout-show-promise/ https://rsolomakhin.github.io/pr/ko/show-promise-unsupported-method/