Why does the Nostromo have a self destruct device?

Maybe, maybe not.
What if they were attacked by an alien spaceship they knew
nothing about ... and they wanted to be sure not to reveal
the location of Earth or any technology or intelligence.
But the main necessity of a self-destruct sequence is to
up the suspense at the end of the story!

It's been a very long time since I watched this movie, but wasn't the self-destruct caused by shutting off coolant to the reactor, leading to a nuclear explosion? An oil tanker wouldn't have a self-destruct device, but if it was powered by a nuclear reactor and you shut off the coolant you would get very poor results...

they did have protocols for encountering alien life or foreign substances. the crew broke quarantine by bringing the guy back. maybe it is self destruct so they can't bring anything back if they did encounter something

In our present times only military ships have scuttle procedures as its considered a high risk if the ship were to fall into enemy hands. While true in our current times industrial ships don't need scuttle procedures we don't know what kind of risks ships like the nastromo would have if they fell into the wrong hands so I can imagine it woulden't be to difficult to assume enterpises of the future implementing scuttle procedures.

And to be fare the Nostromo didn't have much of a self destruct device. They merely turned off the cooling unit for the reactor with the computer being configured to not allow reactivation of the cooling unit if the countdown went past 5 minutes. The fact that the ship exploded rather then just melting down was just eye candy for the audience.

I'm pretty sure Ripley and co. sabotaged something with the coolant in order to initiate a big bang.

The plot required it. Do navy ships have destruct? Do military planes ? No from what I read

Weyland-Yutani probably have two (practical and shady) reasons for including a self-destruct:

1. They don't want rival companies hiring literal space pirates to take over ships and allow cargo and W-Y ships into the hands of their rivals. I could see that in the military-corporate world established in Alien. So, whether it's space piracy or the really cutthroat world of trans-solar system corporate (literal) warfare, W-Y might put in a self-destruct.

2. They are up to some ILLEGAL stuff and the self-destruct is to prevent any boarding, raiding, or monitoring of any kind. It's possible there are self-destruct protocols to prevent the Feds from seeing what really goes down in a W-Y ship. Who knows? Maybe not even the crew...but it wouldn't surprise me if there's a bunch of illegal stuff going on.

Speaking of...

The Nostromo crew might've made it themselves. Parker and Brett are exactly the kind of people who might be smuggling a little extra cargo for a little extra cash. It wouldn't surprise me if a lot of the Nostromo's crew could be talked into looking the other way on this stuff and/or having side deals of their own. They might have rigged up the self-destruct for those reasons. Heck, because it's a reactor coolant thing, it might even look like an accident to any investigators...

Of course, it's possible the reasons are more legit:

-Alien invasion might have been anticipated. The space jockey proves that this universe isn't "humans only". We only *know* about the xenomorphs, but maybe there are others...

-Space plague? Maybe it's possible for cargo to become contaminated, and it needs to be fried.

And, finally, maybe there isn't a reason. Maybe the engineer who designed these vessels put it in as a joke. He watched too much sci-fi and reasoned, "Every good starship has a self-destruct," so he slapped it in. It didn't get noticed because the ships are so huge, anybody reviewing it likely got bored by page 1,350 of the design and operations manual...