Why would HVAC system need an image sensor? Trane International's patent application US20140217185 "HVAC System With Camera and Microphone" by Kirby Neal Bicknell explains that the right question is not "why" but "how many image sensors":
"An image data set captured by the image sensor or an audio file captured by the microphone may be searched for the presence of predetermined conditions to reconfigure user settings. For example, if User A likes the room kept at 65 degrees in the winter, while User B likes the room kept at 72 degrees in the winter, an HVAC controller may recognize the presence of either User A or User B to change the default settings to the preferences of that particular user. In addition, the HVAC system controller may be programmed to prioritize one user over another user.
In another example, an HVAC system may be configured to cool a house when the temperature rises above 72 degrees. However, if an image sensor attached to an outdoor unit captures an image of User C or User D in the back yard, the HVAC system may override the default settings and suppress the cooling operation until User C and/or User D leave the back yard. This may be done to suppress the noise generation by the outdoor unit, or for various safety considerations. If User C is a small child, the operation of the outdoor unit with a high speed fan may present a safety consideration. If User D is typically present to mow the yard, presence of this individual may be a condition to trigger shut down of the outdoor unit to avoid grass being sucked into the outdoor unit, and clogging the heat exchanger."