Why should magicians and witches study astrology? Because it is damn useful if you are working with either practical magic or spiritual development.
Now if you believe that rocks, bits of ice, and hot balls of super-heated gas have no effect upon you, then you can skip the rest of this and safely leave astrology out of your studies....because nothing I say will convince you otherwise.
And there are a lot of people who are just looking for an excuse, so that they do not need to study astrology. After all, astrology is complicated enough that you can spend an entire lifetime studying it; and who has that amount of time when you are busy trying to knife your way to the top of the food chain? Won't it just be better to study some form of chaotic new age woo woo that does not give any credit to the whole astrological nonsense?
And if you can reproduce the effects of astrological magic without resorting to the use of astrological magic, then good for you. Go forth with your bad-ass chaotic new age woo woo-ist self.
But some of us find that astrological based magic works pretty well.
There are two underlying assumptions beneath astrological magic. One is that the universe was set in motion by a divine force, and that there is a plan for the universe with the planets and stars being hands on a cosmic clock enabling one to figure out the current state of the universe and the people who inhabit it. The second assumption is that the celestial bodies are the bodies of supernatural intelligences who can influence events on Earth.
As such, a study of astrology will boost the effects of any magic cast at the right time and place, the act being imbued with the time stamp of the celestial bodies present in that moment. In other words, the moment that a spell is cast colors the spell, just like your birth moment colors your personality and potential.
And with the right techniques, one can overcome the effects of one's birth-chart with magic. For an instance, a person born under a sun in Virgo can over time become more like a person who was born under a Leo sun---if the magician desired such a change. And someone who had a crippled moon in a water sign could lessen the effect that transits of the moon had on one's daily emotions and reactions.
Now, it should be noted that such changes do happen naturally, but the changes come slowly and only work in one direction as one birth-chart progresses forward. The magician does not have to accept this snail-pace of change. A magician can override this slow gradual development and choose to be a product of an early century if they feel like putting in the work. Or a much later century. One should note that such a grand working would make it harder to interact and understand those of this century, but great mages are never concerned with the real world because of their unique nature and need to be greater than the drooling mass of humanity that surrounds them.
The basic way of working with astrological magic is though symbols. For instance, the fixed stars are associated by tradition with powers and entities, who in turn are associated with names, which are composed of magical alphabets and magical images. By using the images, one can use the forces of the energies of the celestial entity to work change in oneself and the world.
For those who are curious of what such symbols would look like, just take a look at the cards of a Tarot deck. Thanks to a great magus in the past, the stars are connected with the Hebrew alphabet, which in turn is associated with the Tarot cards. This is why one can both study astrology with a Tarot deck, as well as perform magic with a deck of cards.
So yes, the study of astrology is a long and hard process, but there are reasons to undertake it if you are a witch or magician. Bottom line: it is damn useful.