Fate is an anthropomorphization of the sum of things that happen in the universe. Does it exist? It exists the same way the universe exists. Can it be known or understood? Partially, maybe more completely if we discover a coherent Theory of Everything. Does it have a will? Not in the way that we generally think of will, but it contains the sum of all wills in the universe, and there are sometimes local convergences of minds that produce emergent effects that feel like more than random noise, because they are.

Is it a useful concept? I think it's a prototype tool our ancestors came up with for framing cosmic cause and effect, which is useful in some circumstances. I think it's too crude a tool, and it's more useful to think about cause and effect more locally because most of the time, local effects dominate outcomes and when they don't, it's extremely difficult to make useful inferences about outcomes because we can't really think about the whole universe all at once. That's why invocations of fate are usually associated with throwing up one's hands in despair and/or accepting whatever Fate has thrown at you.

Fate is a (non-comprehensive) model of the entire universe. The universe is chaotic and complicated, full of exceptions and apparent contradictions. It is, as far as I know, impossible to predict. That's why simpler models are helpful, even if they are incomplete representations of the universe, because they allow us to select the most influential factors of the universe and study them in isolation. Fate is overwhelming in scope, and we usually feel helpless in the face of such cosmic forces. It deters us from action.

I could go on, I think Fate as a concept, how we use it to influence ourselves and each other, is enormously fascinating and important to how humans make decisions in their lives. But I think I've hit a natural stopping point and to go on would grow too repetitive without more careful editing.