The way the dash camera works in low power mode is that it is effectively “on”, but not writing to memory. The write to the SD card is energy expensive, so it just puts the recording into RAM, analyzes it for motion or impact, then dumps it from RAM. The RAM capacity is 30 seconds of data, I believe. If motion or impact are detected( depending on your settings), it writes to the SD card.
To save even more energy, the radar unit is used. The radar uses LESS energy than the camera does in low-power mode. The camera is effectively “off”, no data stream is being analyzed in RAM, and the radar determines when to wake up the camera for recording.
The radar wakes the camera up when it detects movement, and the camera starts putting a video recording into RAM, but the camera has to decide to write that data to the SD card or not. Since the radar won’t detect people walking by, only things like vehicles passing, it might be viewed as pointless to save a clip every time a vehicle drives by. Instead, it uses the accelerometer in the camera to ask if the detected vehicle strikes your car with an impact, and then it would save the RAM buffer to SD card.
Really, there are few use cases that I can see for the radar unit. One is where you store your vehicle in a parking garage for long periods of time. This might be if you live in a city and use public transport all week, and only take your car out on weekends. Another might be if you regularly leave your car at the airport for days. In the garage case, you can back into a spot and the radar can detect all traffic in front of you. At the airport, you have to be selective about where and how you park, but there are usually lots of options. In these cases where the car is infrequently used with long periods of inactivity between and while leaving it in public, the radar unit can sip juice off the battery to extend parking security.
For the people that use their car regularly, recharging their battery every day, the radar doesn’t make much sense. In this case regular parking mode would be fine, or adding a battery backup would work better if you anticipate needing parking security over a weekend where you aren’t using the car, such as if you park in the street.
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Powering cams when parked will always be an issue though, so until someone develops a 'magic processor' that uses almost no power we're going to have to choose between parking time and performance, or add another powering scheme to our cams 
Always a compromise in there somewhere....