Dr. John Montague was a doctor of philosophy; he had taken his degree in anthropology, feeling obscurely that in this field he might come closest to his true vocation, the analysis of supernatural manifestations. He was scrupulous about the use of his title because, his investigations being so utterly unscientific, he hoped to borrow an air of respectability, even scholarly authority, from his education.
He hoped to look more respectable than he felt. That's to say that he tried to use his education and title of "doctor" like an impressive suit of clothing that didn't really belong to him. Inside, he felt like an imposter who didn't really merit the respect of the public. He wanted to look the part.