What is destined will happen. Then what is the purpose of astrology. We all have to go through this life-cycle dependent on the influence of planetary positions at birth, called prarabdh. This is dependent on the accumulated karmas of past life/lives and therefore there can be a view that prarabdh is deterministic.
Karma siddhant suggests that karmas of each moment get added to give the accumulated results of kriyamaan karmas or the ongoing karmas. It is here that the role of astrology emerges to give the guidance or margdarshan. Fate is not deterministic. It is what we make with our committed involvement. Astrological guidance can therefore change the destiny for now or for future.
The purpose of astrology is to provide that guidance -- what to do in a particular situation. Forecast never was, and will never be the purpose of astrology. By taking a forecast no one can get any advantage. But guidance based on those indications, can alter the way we think and act, and can change our existence for the better.
Who can provide the right guidance depends on the experience, conditioning and honesty of the astrologer. One who tries to provide magical forecasts and magical solutions to problems may have some hidden selfish motive. Lies may seem very promising, but can misguide you, truth may seem harsh, but has its own influence. That is what we have to understand.