Mark 5:13

‘And the unclean spirits came out and entered into the pigs, and the herd rushed down the slope into the sea, in number about two thousand. And they were choked in the sea.'

The number of pigs confirmed the multiplicity of the evil spirits, and their behaviour confirmed to the watchers, including the man, (and how important psychologically that was), that the evil spirits had really gone. Now he could begin his life again. And the evil spirits were no longer there to trouble man nor beast. They had disappeared into the sea, into ‘the deeps'. We are possibly to see by this that they had gone to the Abyss. Alternately they might have seen the deeps as their home.

On the other hand it is possible that Mark 9:22 is suggesting that the evil spirits could have themselves been responsible for the demise of the pigs, possibly in order to be free to menace others. We can compare here Luke 11:24. However, their departure into the sea might suggest otherwise. It would be foolish to dogmatise.

The question may finally be asked, why did Jesus pander to them at all? While again it would be foolish to dogmatise it is probable that He wanted the man to recognise that he was getting a complete deliverance, while at the same time wanting His followers to recognise His supreme authority, even over thousands of evil spirits at one time, and that Satan was truly bound. And, to accomplish that, all that happened was necessary.