In Alien, the crew, finding themselves unable to kill the titular character, set the Nostromo to self-destruct and attempt to escape in the shuttle.
But why destroy it? It actually cost extra effort to destroy the ship. If they just left, the Alien couldn't get at them, and they wouldn't have destroyed a perfectly good ship with its extremely valuable cargo.
The crew of the Nostromo were guided by fear that if they simply abandoned ship that the Xenomorph would successfully infect any crew that came to salvage the Nostromo and that Weyland-Yutani would continue with their plan take it to Earth where it might cause an outbreak.
Ash (in the novelisation) lays it out quite nicely;
'I cannot say. There is a distinct possibility the alien will successfully infect the boarding party and any others it comes in contact with before they realize the magnitude of the danger it presents and can take steps to combat it. By then it may be too late.
'Thousands of years of effort have not enabled man to eradicate other parasites. He has never before encountered one this advanced. Try to imagine several billion mosquitoes functioning in intelligent consort with one another. Would mankind have a chance?
'Of course, if I am present and functional when the Nostromo arrives, I can inform the boarding party of what they may expect and how to proceed safely against it. By destroying me, you risk loosing a terrible plague on mankind.'
Although Ripley's reasoning isn't laid out on the final cut of the film, we do get an idea of her motivation from the original "Starbeast" version of the script. In short, the alien needs to die.
HUNTER: Blow the ship up?
ROBY: And the creature with it. We can make it back to Earth in the lifeboat.
Ok, so after reading through the script I found here, at one point a plan to just abandon the ship was suggested and promptly thrown out.
Ripley: Unless someone's got a better idea about dealing with the Alien, we'll proceed with Dallas' plan.
Lambert: And wind up the same way. No thanks.
Parker: You've got a better idea.
Lambert: Yes. Abandon ship. Take the shuttle craft and get the hell out of here. Take our chances on getting picked up later.
Ash: You are forgetting something. Dallas and Brett may not be dead. It's a ghastly probability perhaps, but not a certainty.
Ripley: Ash is right. We've got to give it another try. We know it's using the air shafts. Let's take it level by level. This time we'll laser seal every bulkhead and vent behind us until we corner it.
It isn't until later in the movie once Ash is revealed to be an android and that his mission is to bring the alien back alive, while the crew of the Nostromo are all expendable that it's decided to self destruct the ship, ensuring the Alien is dead and Weyland-Yutani never gets their hands on it.
