Imagine you opted for the cheap $500/month plan. You might have to pay 50% or 100% of all costs until you reach your maximum, which might be 5 digits and resets at least annually. Don't forget the "out of network" vs "in network" separation/cost/caps, where your physical hospital may be in network but a provider that works there is billed out of network.
See also: https://en.wikipedia.org/wiki/High-deductible_health_plan