If a hospital routinely runs at 75% capacity in their ER and ICUs, and they suddenly get an influx of covid cases... it will also swamp them. So, for instance, if a hospital has 100 beds that can treat critical covid patients... but they routinely use 75 of them for other conditions (heart failure, stroke, severe car accidents, countless other emergent conditions/events).... if they suddenly get 40 covid critical patients, they will be beyond capacity. Even if they only have 40 covid patients with 100 beds.
The care for other emergent conditions will also suffer as beds fill with critical covid patients.