There is not necssarily "evidence" for this, and it would likely be unethical to have a controlled trial - but basically there are a lot of patients whose condition would significantly deteriorate if they were not seen outside of the normal working hours.

you could examine the differences in length of hospital stay, incidence of admission to icu for respiratory complications from wards, etc between hospitals that have/do not have on-call physio services but there would likely be too many potentially confounding factors, meaning any results would be nigh on meaningless.