poppers are (in USA) a legal way to get high. They don't last long, and they also loosen the smooth muscle tissue in the anus, so they're popular for bottoms to use in anal sex. For a lot of people, the particular high of poppers (especially with weed) makes them super horny too. Popper trainer videos are generally there to make the experience of making poppers more fun; getting high with them makes the stories seem to come to life and makes it easier to imagine being the characters in the story. To each his own!