Some things are fixed and some aren't. I can only guess at what determines which is which, but I have some ideas.
I've had visions of the future that came true no matter what I did, and I've had other visions that I was able to change. Again, I can only guess why things go one way or the other.
I keep a dream journal and every dream I have about the future I write down. I know they're future dreams because they have a certain energy signature to them. It's like a memory you haven't lived yet. A memory of the future. You are remembering something your physical body hasn't experienced yet. You can explore the future memory just like a past memory, but it has limits because it lacks experiential understanding. You don't understand the memory like you would a past memory. You haven't "been there" yet, even though you're remembering it like you were there.
I've had dreams come true years later. It's weird because the dream could be about something utterly trivial. A recent example was me getting into the car of an old woman who was driving me to a supermarket. In the original dream I didn't recognize the woman or the place I was in. I had this dream in 2018 while I was so doped up on morphine I could barely stay awake. I woke up wondering what the hell that was about because it seemed so incredibly mundane and I wondered how my mind generated these unknown images.
Just a month ago, that scene played out. It was my bf's mother, a bf (and his mother) who I never met yet, in a place I had never lived yet. I went to her house to pickup some mail and she offered to drive me to the grocery store where I was headed next, because she needed to go there herself anyway. Same car, same woman, same scene, all of it. It felt almost like a deja vu, except not quite... which made me suspect that it was actually a dream. Sure enough, when I did a word search of my journal for "grocery store", I found the entry. In the dream, I couldn't understand any of it. I had to live my way into that moment in order to understand it.
Seeing the future is not all it's cracked up to be. Most of the time it's utterly useless... like, why bother seeing this? But that question is irrelevant because it's like saying why have a memory of the past? Or why see what I'm seeing now in the present? Why is irrelevant. It's just nature. As the Oracle said in The Matrix, "you can't understand a choice you haven't made yet." The future is almost always out of context, unless you get one of those very rare crystal clear prophecy type visions where it's spelled out for you. In those cases, it's almost like a warning that you're meant to fully understand so that you do something about it. But a warning from whom? Or what? Other times, it's an image outside of time that has no apparent context, and you have no idea wtf to do with it, and in fact there's nothing to do about it... and when you eventually live your way into it as the future aligns with the present, you realize it was pointless.
I don't know how all this works... but time is clearly not just linear. It's non-linear as well. Wind blows. Rain falls. Sun shines. Time and memory sometimes do these weird things. It may have no purpose at all.