Rapid eye movement sleep (REM sleep or REMS) is a unique phase of sleep in mammals and birds, distinguishable by random/rapid movement of the eyes, accompanied with low muscle tone throughout the body, and the propensity of the sleeper to dream vividly."
[ Source Wikipedia, link below ]
First described in the mid 1950s, REM sleep is known to be an essential component of healthy sleep. The questions of why the eyes move rapidly, what regulates REM episodes, and what are the precise functions of REM sleep are yet to be answered.
There are currently at least 8 competing theories.
Some researchers maintain that has no purpose at all, and simply results from random brain signal activity.
Note: Sleep periods without REM is termed 'NREM' (non-REM) - the purpose of which is also unclear (see Sleepplugin-autotooltip__plain plugin-autotooltip_bigSleep
"The purposes and mechanisms of sleep are only partially clear and are the subject of intense research"
Source Wikipedia"
Although extensive research in humans and animals has shown unequivocally that sleep is essential, the reasons why it's required are as yet unknown. Long term sleep deprivation not only severely impairs cognitive and motor skills, it's now been shown that it can cause physical damage to brain structure - and, in extreme cases, irreversible damage.)
Further reading : Wikipedia
Also see Dreamingplugin-autotooltip__plain plugin-autotooltip_bigDreaming
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"The content and purpose of dreams are not yet understood, though they have been a topic of speculation and interest throughout recorded history.
Source : Wikipedia"
Not only is the function of dreaming unknown (if there is one), but there isn't as yet an agreed definition of what it is.