Need help understanding the “foresight ability”

I need help understanding the foresight ability in warhammer. I remember seeing a comment somewhere where the person said "this person would always win because they can see the future".

But isn't the future like "malleable" in a way? I mean just because you can see the future doesn't guarantee it would happen, since they'll get visions of multiple outcomes and such. Am I correct or am I wrong?

Because if the future is set and stone then wouldnt those that have the ability just always win a war then and never lose?