There’s a saying attributed to various people but mostly to Anton Chekhov: “If in the first act you have hung a pistol on the wall, then in the following one it should be fired. Otherwise don’t put it there.” I’m pretty sure I read it as a Chandler quote about putting a gun on the mantelpiece, but the meaning is the same. If you introduce something into a story that is definitely not just set dressing, you should use it.
My problem in this respect is that I have a gun on the mantelpiece and another on the hearth, there’s also a knife tucked into a book shelf, an AK47 resting on top the TV and some grenades hidden behind the sofa cushions. Every time I come up with a bright idea and introduce it to the story, it usually get complicated. Why? Because this is science fiction and the two guns talk, belong to a psychopath, and hate each other. Because the grenades are a conscious, slightly insane hive mind, and also belong to the psychopath, or so he thinks. Because the AK47 is controlled by a genocide surviving conjoined alien entity with plenty of resentments of its own. And the knife? Well it’s just a knife but I’m pretty damned sure one of the nutjobs in the room will use it.
No matter. I am in ‘just write it Neal’ mode and I can maybe use the knife myself later to excise the proliferating plot threads. I’ll then place them in my BitsSF file and at some later date turn each into a short story.