In short
Since merely human exorcists relied on their formulae being heard by the victims of demon oppression, they could not help a deaf man. Matthew’s word behold
implies how unusual this case was.
Earthly exorcists are unable to treat the man who was deaf-mute, for because of the man’s deafness, the demon was isolated from their exorcising formulae. Matthew 9:32 directs our attention to the exceptionality of the case by emphatically introducing it with the word behold.
1
32 As they were going away, behold, a demon-oppressed man who was mute was brought to him.