To the uninitiated, there may not seem to be much difference -- after all, heroin and acid are both hard drugs. But although some of the themes are similar, for example, the cautionary tale, the touting of freedom of choice, and the hedonism of drug abuse, heroin and acid represent very different worlds.
One of the main aspects of the LSD intoxication experience is the perception of visual distortions and hallucinations that are experienced by users. Film provides a medium through which film-makers can attempt to portray what this phenomenon is like to people who have never experienced it. While many acid movies were made in the heyday of LSD use in the 1960s, the more recent advent of special effects has created many more opportunities to re-create the experience.
If you are curious about trying LSD, you could think of these movies as a way to gain all the insight you need to know what the experience is like. That way, you can enjoy the visual distortions, without having a bad trip, the effects of which can last for years.