I mean, is it meant to nullify other belief systems? Is that the purpose?
I don't think so... like much of philosophy it doesn't have pragmatic implications beyond being a mental whack-off. I think the only belief system it nullifies in a broad sense is a non-deterministic one. There is theistic determinism, non-theistic determinism in many flavors. It certainly has some implications for other belief systems though.
To me, it comes down to: If you have a powerful enough computer that can keep track of all of the atoms in the universe, can it predict what will happen in the future, and further, what decisions you will make?
Some say quantum mechanics precludes this, but others say QM only affects determinism up to a certain scale and we still have 'determined probabilities'. It really depends on what your interpretation of QM is (for example whether or not the wavefunction is objectively real).
Then, as for decision making, it depends on whether or not you think the mind arises sole from the brain. If so, I don't see how you can have free will, as your brain is nothing more than atomic interactions that can be predicted. If your 'thoughts' and 'contemplation' arise from atomic interactions that have been occuring in a long chain of cause and effect, even when you 'choose' to change your mind you would have done so anyway. If you believe in a dualistic mind that arises from elsewhere (such as many theists do, soul, etc.), there are cases for and against determinism.
I said in the beginning that it is mental masturbation from a pragmatic standpoint, because even a staunch determinist doesn't hold that a lack of metaphysical free will changes how you live your life in any way. I suppose in a certain light it can seem like a depressing concept, but it need not-
I don't think we will ever build that universe sized computer, so we will never actually determine everything.