Ah, yes, of course. A UA plane that arrives in CDG at 9:30 pm is not going anywhere that evening. The best one could hope for would be a 7 am departure to the US.
However,
Flights to/from South America tend to be red-eye both ways and thus planes remain unused for 10-12 hours in places like EZE, GRU and such. If UA can afford to keep a plane unused for 10-12 hours in Argentina, why not also in France?
For US airlines at least, easy, connecting traffic.
Some US airlines like AA, has maintenance facilities in South America to alleviate the idle time.