Alarm systems use the "End of Line" resistor concept. The alarm triggers when a sensor is opened and current flow stops. Closed is ambiguous because it could mean power is flowing through all closed sensors OR there is a short (alarm panels handle shorts). A resistor allows the panel to consider 0 Ohms a short and some specific number of Ohms as closed and working. It apparently also provides a way to detect tampering of the wires.

https://www.alarmsystemstore.com/pages/all-about-resistors