Offtopic, but this would have two functions. First, it could serve, at least to some degree, as a general house occupancy status. If the car is in the garage, then someone is either at the house or within walking distance. Or on vacation. Or picked up by friends. Still, it could serve as a "backup" as it were, for things like auto-unlocking the door or something to double-check for sounding a security alarm
edit: In line with that, if the car is in the garage, and someone attempts to open the garage from the outside, there's something wrong. Then if, for instance, someone then enters an invalid lock code, that'd be enough for an alarm. /edit
edit2: Thinking this through, since it'd be easy to drop something in front of beam sensor, trying to account for that within a security system would be overly complicated for the very limited benefit. /edit2
The bigger reason would be a parking-assist stop light system. Like this (random product and vendor as just the first one I found). edit: A true "stop light" would need multiple sensors, but I'm actually thinking more of a lighted "stop sign".
I'm pretty sure a parked car would not in fact be caught by a motion sensor. If it could only sense arrival and departure, then you're back to not being able to tell the difference between a person or car (except maybe if you could turn the sensitivity wayyyy down).
edit: And not actually know if the car is there or not, just whether it had arrived or departed, though I admit I have a beam sensor at the door of the garage.