I think you misinterpret what the Steel Dossier is...
The Dossier is a load of third hand, fourth hand.... information
At heart is confusion over exactly what the Steele dossier actually is.
thehill.com
The information in the Dossier is more about people are saying about Trump rather than it being evidence in itself...
Example:
The dossier said the two Russian agents were talking about Trump having hookers in a Moscow hotel... The report states that someone over heard chatter of the two agents talking... This is not to say what they are saying is true, but that if the first agent is to be believed (thats a big if) then it is believable that one Russian agent could say this to another agent and the second agent isn't saying that is bullshit...
That is what the report is actually saying. Is that true? We have no evidence to say if it is true or not. That is the nature of these types of intelligence reports, they are not legal.
Do investigators use them to investigate? Yes, this is where they start but it usually just gives them a direction...
The Steel Dossier was not the reason Trump Campaign was investigated, at this stage you must understand that. The Steel Dossier was used during the investigation because it could be an indication of what was going on but it would be treated like unsubstantiated chats...