You can't do good astrology if the underlying math calculations are wrong. Most mainstream Western astrology uses Placidus houses and then places planets in signs within those houses. From the planetary placements aspects (angles) are calculated.

The sign that a planet is in, which house, and aspects between the planets are what enables astrological fortune-telling and prediction (the forecasting aspect of astrology is technically known as transits and progressions, this is the "predicting the future" piece).

The bedside manner of the astrologer obviously has an enormous influence. One astrologer will look at a chart and treat it as a fixed "you are this way because of your chart" as if you can't change. A more thoughtful (read: actually good) astrologer will treat the chart as an evolutionary blueprint for growth and change. In many ways astrology is really just a different version of psychology and counseling.

A great book to read if you're open-minded but also don't want a bunch of "woo" is to check The Inner Sky by Steven Forrest. The first chapter of the book is called "Why Bother?" For what it's worth I had my chart read by Steven and it has really helped me in my life.