Final answer:
The best indication that dreaming has a biological function is the REM rebound observed in mammals, which implies that REM sleep plays an essential role in learning and memory, as well as emotional regulation.
Step-by-step explanation:
The best indication that dreaming serves a necessary biological function is provided by the fact that most mammals experience REM rebound when they are deprived of REM sleep and then allowed to sleep without disturbance. This phenomenon demonstrates the homeostatic regulation of REM sleep, indicating its biological necessity. Biological researchers believe that REM sleep may be involved in processes related to learning and memory, as well as emotional processing and regulation. This essential role underscores the adaptive and restorative functions of REM sleep, making it more than just an arbitrary feature of mammalian sleep.