All authors have a perspective. All authors have reasons for writing on one topic and not another. It is important to know an author's point of view so you can understand where the author is coming from. Information is always colored by its creator.
There is nothing wrong with an author having a point of view. However, sometimes particularly strong points of view create extreme bias.
Extreme bias is when information is distorted or suppressed in order to force the reader to reach the same conclusion the author proposes. Biased information is intended to manipulate the audience. You should always take into consideration an author's point of view when evaluating information, but you should beware of extremely biased information.