Why is the Steele dossier so important? There are two reasons. 

Firstly, the document was paid for by the opposing political party, the DNC, and the candidate running against Trump, Hillary Clinton. 

Secondly, the claims in the document were wild and have been unverified. There was no fact check on the document and the allegations within are unsubstantiated. To put it lightly, there is no way to know if Steele simply made up every detail of the document.