An advertising network sells screen time across many venues as one media buy, so an advertiser reaches a hundred locations without negotiating a hundred contracts. In digital out-of-home this increasingly happens programmatically: inventory is bid on in an auction, the same way display advertising works online.
For a venue owner it turns screens from a cost into revenue: unsold time is filled by the network and paid for. For an advertiser it makes out-of-home buyable in campaign-sized units rather than annual site leases.
The caveat for venue owners is control. Joining a network means content you did not choose appearing in your space, and the exclusion controls are usually category-level rather than brand-level. A retailer can typically block "competitor category" but not a specific competitor, and finding out where that line sits is a contract question, not a technical one.
The measurement caveat is bigger: out-of-home impressions are modelled from footfall estimates rather than counted like a web page view. The number in the report is a projection built on assumptions, and the assumptions vary by network. Ask what the model is before comparing two networks' numbers.