“Digital signage” refers to screens showing ads on billboards, announcements on public transport, menus in fast-food restaurants, and the like. Those aren’t cheap devices: consider that not so long ago you’d encounter humongous plasma displays there. Using a full (industrial) PC to drive one is totally normal, and the cost of an RPi is likely negligible. Don’t know if an RPi is well-built enough though, as some signage installations may need to exist in pretty hostile environments (vibration, dirt, EMI, etc.).