If a witness is presumed hostile, or declared hostile by the judge, it changes a few things. The most important is the way that the attorney is allowed to ask a hostile witness questions. Normally, an attorney cannot ask its own, non-hostile witnesses leading questions. A leading question is one whose answer is presumed in the question, often answerable with a simple "yes" or "no."
For example, a non-leading question would be, "What color was the light?" A leading question would be, "The light was red, wasn't it?" Leading questions tend to coach the witness because they tell the witness what to say. Non-leading questions force the witness to come up with his or her own answers.
Generally, an attorney may ask leading questions to hostile witnesses only. In normal trials, leading questions are reserved for cross-examining the opposing side's witnesses; all witnesses called by the opposing side are presumed hostile. But, when a judge determines that a witness is hostile, then the attorney who called him may use leading questions to help focus the hostile witness' testimony.
Another effect this has is that it can make the scope of a direct examination difficult to manage. Normally, an attorney asks his own witness non-leading questions, and the witness answers accordingly. But, with leading questions, the attorney is effectively dictating the scope of the testimony because he limits it in the very structure of his questions. To make sure that the testimony stays on topic, a trial judge must use his discretion to limit the attorney's questioning.Last, determining that a witness is hostile is important for impeachment purposes. In some jurisdictions, a party cannot impeach his own witness, unless that witness is declared hostile. This arises in criminal cases sometimes where a witness gives pretrial statements about a crime, but then changes his testimony at trial because he is scared. In that situation, an attorney wants to show to the jury that the witness was telling the truth before, but is lying now. In jurisdictions that limit impeachment to hostile witnesses, the attorney would have to establish that the witness is now hostile before evidence of the pretrial statement could be introduced. But, as I mentioned before, declaring a witness hostile is rare. Dramatic, but rare. Most witnesses testify truthfully, and are inherently biased toward the side that called them. Only in rare circumstances does a court need to step in to declare a witness hostile.