For most of its history, out-of-home was bought by location and sold by estimate. You chose a board, you paid for a month, and you accepted a traffic count as evidence that people saw it. The medium was powerful and almost entirely unaccountable.
Digital out-of-home changes the mechanics without changing the impact. Screens across billboard networks, transit systems, airports, retail environments, gyms, and venues are bought as impressions through the same demand-side platforms used for display and video. Delivery can be scheduled by hour, triggered by weather or an event, and changed the same afternoon.
The more important shift is measurement. Mobile devices present during a play window can be identified, held as an audience, and followed into site visits, store visits, and conversions. That is what turns DOOH from a brand gesture into a media line that can be defended, and what lets it sit inside an omnichannel plan rather than beside one.