I wasn't in the room when the decision was made, but my understanding from reading both the Steele Dossier and the published Mueller report is that Mueller drew considerably from the Steele Dossier as a way to understand where to consider targeting the investigation early on, and then expanded elsewhere after they had identified that most of it was not actionable intelligence. Mueller's investigation ultimately resulted in several convictions, so where there is smoke there is fire, but in this case the fire was burning oak wood and the smoke smelled like mesquite.