How might a speech or a document be weakened by unsubstantiated opinions? How does a lack of evidence affect the effectiveness of an argument?

A lack of evidence in an argument makes you look untrustworthy, and unreliable. People need evidence to trust in your words but if you have none then why are you even arguing? A speech or document can be weakened by unsubstantiated opinions because you need facts, not opinions. If your speech is solely founded on opinions then you seem untrustworthy.