To start it helps to have a dream journal alongside your bed. I cannot count the times I've had a dream that seemed likely to be book material but after breakfast I have no idea what it was about. Write it down even if you are sure you won't forget.
If you sleep with someone, telling them the dream is helpful also. I don't know why just remembering it isn't enough but for me it isn't. Once I have verbalized it, I hold it better.
Often dreams are filled with symbolisms which may not work for the plot of a book but might be useful to flesh out a character. I've used some of my dreams as a dream in the book when the symbolisms fit the storyline. I've also made up dreams that enriched the foreshadowing of what's coming.
It's interesting to wonder about what makes a dream happen especially movie dreams. I've had some where I know I can't use them as they aren't in my genre, but they are worth writing down for a possible future where that changes. If nothing else, they are as much fun as watching a movie but where I have some control over the outcome as I find I can do lucid dreaming to take a dream where I want it to go.
When dreams aren't about writing in a book or creating a character, they can have value for our personal lives based on what our subconscious might be trying to tell us. I go to a dream site for ideas on what a color might mean or perhaps an insect that is not normally found in my life.
My favorite site for that is Dream Moods.
So for instance when I had the dream where you could tell when someone was a bad person by the translucent color red on their throat, which not all people could see, I looked it up. The color red represents, "an indication of raw energy, force, vigor, intense passion, aggression, power, courage, impulsiveness and passion." That makes it a logical color for the warning but what about it being on the throat?
Although the dream dictionary didn't give me the answer to that, my own interpretation is perhaps it relates to not trusting what others say or their words. It wasn't my throat but someone else's. We know we need to be wary of what we trust in the news or from leaders. That dream might not have related to any possible story (unless I get into writing horror which I don't see happening) but to my own life.