A concept that needs proof before the money arrives.
The link budget closes on paper. The battery math works in a spreadsheet. The customer, the board, or the investor wants to see it run, and the date is closer than the team.
A proof of concept built by one engineer in weeks: working radio, sensor, or controller on the bench, measured against the numbers that were promised, with a design that carries straight into the product. The demo is the deliverable, and it is not disposable.