There is no need for the imaginary demons invented by Christian sects because men are evil all by themselves.
The demons mentioned in the bible are simply personifications of all sorts of illnesses and diseases men encounter on their way to death. Sin came into the world by one man and death by sin. Which means that Adam's sin has brought death to all.
Anyone who knows the Scripture ought to know that sin and the flesh is personified. It is personified as the Devil and Satan because it was the lust of the flesh which caused the first sin and has brought death to everyone.
Therefore, demons are also a personification of what the lust of the flesh has caused.
That means that when a demon is cast out, the person who was suffering some form of malady, was cured of that illness.
For example, after Jesus had transferred the illnesses(demons) of a man into the swine the man was said to be restored to his right mind. The man was obviously suffering from some sort of mental illness which caused him to behave like he did. There are many such cases even today. However, advances in medicine have been able to make these people harmless to themselves and others.
Does medication cast out "fallen angels"?