When you connect a remote interface device to your LAN, you can monitor environmental conditions or the performance of unattended equipment and operate equipment by remote control through your network. Many possible applications for this kind of networked remote activity are available, including the following:
Monitoring the temperature in an equipment room, a freezer, or "cold box"
Measuring and monitoring air or water temperature, wind speed, water flow in a stream or other body of water, and other environmental conditions
Monitoring power levels
Monitoring and responding to alarms
Turning AC or DC power off and back on again in order to restart "hung" equipment
Monitoring normally open or normally closed intrusion alarms (such as open doors or windows)
Remotely controlling and adjusting isolated equipment
Tracking any other condition that can be measured or monitored with a digital sensor or operated with a relay
Most of these applications are more practical for relatively large businesses and government agencies that operate in several locations (such as a county- or statewide system of radio transmitters or a scientific study that allows researchers in multiple locations to track environmental conditions through the Web), but some can be adapted for a home or small business network. For example, a water detector in the cellar might trigger an alarm or send an email message, or an intrusion alarm, a fire alarm, and a remote temperature sensor in a barn could all connect to a network on a farm through a Wi-Fi link to a computer or other monitoring device in the house. Or with the right kind of equipment connected to your home network, you could send an instruction through the Internet from your workplace in case of a sudden snowstorm that would turn on heating coils embedded in your driveway and melt the ice before you arrive home. For that matter, you could even operate a model railroad through your network, but you'll need somebody near the tracks to take care of derailments and other scale-model disasters!
Remote controls, thermostats, sensors, and monitors are specialized devices, but they're widely available through industrial sources and from retailers and mail/web-order suppliers of home automation equipment.
Every remote sensor and control that connects to your network through a wire or cable increases the network's sensitivity to lightning strikes. Remember to use appropriate lightning suppression wherever practical.