I once had a job where, after four months in the office, I still couldn't pinpoint exactly what one person in my office did. She and I would be in the same meetings, but she didn't say or do anything that hinted at her professional purpose.
Not once did we collaborate on a project, though we made polite conversation in the hallways. When I asked my boss about this person's role, I received an eye roll and a shrug. In other words, no one knew what this person with a vague job title spent her time doing.