The Passiv Smart Thermostat optimises your heating depending on external temperatures and weather conditions. In order to ensure that this is done accurately for your location, it uses your postcode to check local weather forecasts.
Your postcode is also used if you have a smart tariff. Certain tariffs, such as Octopus Cosy, can differ depending on region. To make sure that your tariff information is accurate, the Passiv Smart Thermostat uses your postcode to look up your unit rates and timings for your local area.