Luke 8:32

‘Now there was there a herd of many swine feeding on the mountain, and they entreated him that he would give them leave to enter into them. And he gave them leave.'

Nearby was a large herd of pigs. The evil spirits would be aware that to One connected with the God of Israel pigs were ‘unclean' creatures. Thus they sought permission to enter the pigs, and Jesus gave them permission. Perhaps they felt that once there they would be relatively safe from the God of Israel to Whom pigs were unclean. We must consider it quite possible that they thought that they had now tricked Jesus into leaving them alone. They were out of His territory.

But Jesus was perfectly satisfied. This would prevent them entering some other human being (something those who grumbled about it overlooked). It would also be evidence to the man and to eyewitnesses that the man himself had been released. It is also possible that Jesus in His manhood did not actually know what the final reaction would be. There was no outward indication of what would happen, and it is doubtful if the evil spirits were expecting it.

Or perhaps it was done with the deliberate intention (without their realising it) of consigning the evil spirits to the Abyss. It would be to be rid of the evil spirits without them causing trouble elsewhere (when they left the man they would necessarily seek to go somewhere, compare Luke 11:24).