I just want one sensor. Why do I need the Shield Hub and a monitoring plan?
Shield Sensors don't have a way to alert you without the SCW Shield Hub.
In order to last longer, Shield Sensors use a low power method of communication that only works with the hub. If Shield Sensors had wifi or cellular connections in them, they would be dramatically more expensive and their batteries would not last very long.
Here's a direct link to the "Why do I have to have a monthly plan for the Shield alarm or Shield App?" question, which is related.