Why self destruct?

Why does every spaceship in the movies have self-destruct? I get that they have some kind of highly volatile, magical powered core that allows the ship to travel through space in some warp-type way, but why does it need to be blown up? Take The Enterprise from Star Trek, it can eject its warp core if it becomes unstable. So why do you need to blow it up? Even if you couldn't eject it, you are in space, the materials in the core aren't going to harm some planet's environment. Not that blowing up the ship wouldn't spread the harmful components around anyway. Think about every science fiction movie you have seen. How many had a ship, or other vehicle, or even a building with a self-destruct mechanism? I bet it's a high percentage.

Now, I understand it is a plot device to raise tension, but can't we find something else? Please? Or, how about this; the self-destruct has to be ridiculously hard to activate. Like you have to light 10 fuses on ten different decks, but one guy can't find a match, or something. I'm just throwing out something off the top of my head. (See, the fuses would be totally retro. A cool retro, don't you think? No? Just me?)