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Our daydreams are often unrealistic but usually not so fantastical as to be outside the realm of possibility. They usually include situations we might actually find ourselves in and they probably include us doing things we wouldn't normally do. They will definitely include other people reacting to us in an unrealistic way.
Are daydreams part of our mental software that is meant to prepare us for future moments?
Fantasies are meant to be unrealistic. If we ever reach a place where we achieve outcomes we have fantasized about then our brains conjure new and more extreme fantasies.
Are fantasies a part of our mental software that drives us to achieve?
Is there really any difference between daydreams and fantasies?
Should we worry about violent fantasies (of murder, etc)?

Links
https://en.wikipedia.org/wiki/Daydream
https://en.wikipedia.org/wiki/Fantasy_(psychology)
https://www.theatlantic.com/health/archive/2019/01/violent-fantasies-rumination/580858/
https://www.washingtonpost.com/archive/local/1990/06/13/violent-fantasies-common-among-serial-killers/2884a9ca-bc70-4903-b193-ce9d0aa58c48/
http://archive.boston.com/news/nation/articles/2007/04/20/violent_fantasies_not_uncommon/
https://wildabouttrial.com/trial-coverage/gilberto-valle-cannibal-cop-case/