An overnight disruption turns an airline's problem into a lodging problem within an hour. How passengers end up in rooms is a supply exercise that was arranged long before the delay happened.
The inventory is contracted in advance
Carriers hold standing arrangements with hotels near their stations, covering a block of rooms at agreed rates for irregular operations. That inventory is not bought on the night.
The blocks are sized for ordinary disruption, meaning a mechanical issue or a single canceled flight rather than a regional storm that grounds everything at once.
When a weather event cancels dozens of flights, contracted blocks are exhausted quickly and the search widens to whatever the city has left, which is often far from the airport.
Vouchers are instructions to a specific property
A hotel voucher is generally issued for a named property with a confirmation attached, not as a credit a passenger can spend anywhere. The hotel bills the carrier afterward.
Transport vouchers work the same way, which is why a passenger sent to a hotel without a shuttle is usually also given a means of getting there.
Because the arrangement is property-specific, a passenger who books a different hotel independently is generally pursuing reimbursement afterward rather than using the voucher.
The cause of the delay drives who pays
The general distinction carriers draw is between disruptions within their control, such as maintenance or crew problems, and those outside it, such as weather or air traffic restrictions.
Accommodation is far more likely to be offered in the first category. In the second, carriers commonly help arrange discounted rooms without covering the cost.
These commitments live in each airline's published customer service plan and in the contract of carriage, and they differ between carriers and change over time, so the document is the thing to check.
Airport hotels absorb the surge unevenly
A mass disruption is a demand spike arriving after the normal check-in window, when properties are already near full and staffed for the evening.
Front desks handling a bus load of unexpected arrivals slow to a crawl, and housekeeping cannot produce rooms that were not vacated. Availability, not willingness, is the binding limit.
This is why the traveler who books directly from the gate, before the agent finishes making announcements, often gets the last nearby room.
Bags usually do not follow the passenger
Checked luggage on a canceled flight is frequently held in the system for the rebooked flight rather than returned, which leaves passengers overnight without their belongings.
Some carriers provide a small amenity kit for that reason, and some hotels keep a supply of basics at the desk for the same situation.
Practically, the case for a change of clothes and medication in a carry-on is entirely about this scenario, where the hotel is available and the suitcase is not.