Why would the Nostromo have a self-destruct mechanism?

Question

Just been watching the excellent Alien movie and wondered: why would the Nostromo, an innocuous civilian cargo vessel, have a multi-million-dollar built-in self-destruct mechanism?

Answer 1

Traditionally, during war, cargo transported for war effort is loaded onto civilian ships, mostly. What any navy has in it's inventory, in terms of cargo shipping, is basically for peacetime internal needs or specialized units that are part of naval formations (i.e. ro-ro ships, tankers attached to carrier battle groups etc.)

Those civilian ships would be privately owned and operated for the simple reason that no navy has the resources to take them over and run them; also it usually is counterproductive. For example, merchant cargo vessel has only as many crew members on board as is necessary. Navy usually operates with larger complements compared to civilian crews for redundancy.

[See: Everything You’ve Always Wanted to Know about the Merchant Marine But Were Afraid to Ask]

That being said, merchant vessels are legitimate targets in war if they are operated by the warring state or are required to operate within that state's military structure.

[See: Naval Targeting: Lawful Objects of Attack]

So, to avoid capturing ships and cargo by the enemy, they can be (though this is captain's decision, of course - civilians are not military) equipped with self-destruct to facilitate that desire to avoid capture. (the Wartime Instructions for United States Merchant Vessels does not mention scuttling, just rules for engagement to avoid capture; practical outcome of capturing of a cargo vessel by belligerent was scuttling anyway)

Self-destruct as an installation (because it's not a single device) is rarely encountered as it's quite expensive piece of equipment to acquire and then maintain. This is because such installation is expected to work when needed and only when needed - accidental activation is rather irreversible, and that includes all kinds of damage, up to and including combat-related. And, of course, it is designed to be initiated from single source.

For the most part such devices are not found on commercial vessels for the simple reason that they are not needed - practically every ship can (and historically could have been; we know this happened from many records) be scuttled, that is: intentionally sunk. This always was - and is - achieved by either blowing holes in the bottom of the vessel by brute force (i.e. men with axes and hammers), opening specially installed for this reason scuttling plugs or by placing explosive devices in vulnerable places. In first and second cases this was very dangerous operation, so would be done by volunteers, in the last scenario explosives would be produced and placed just before the actual scuttling.

[As mentioned in "The Grand Scuttle", the scuttling crews would prefer to have explosives to open flood valves - due to overgrowth of barnacles on the outer hull and had to remove any controls of those valves on the open decks on the ships. The opening of the valves on the day of scuttling was done by designated crews and hammers, axes and other tools were laid out next to the valves to do the work as expeditiously as possible. Even with all precautions and days of preparation, 8 sailors were killed during scuttling.]

In terms of Nostromo the self destruct caused a meltdown and subsequently explosion of the power source.[See Emergency destruction system Aboard the_Nostromo]

However, in stark contrast to what I wrote earlier, such self-destruct is neither expensive nor necessarily built-in. The multiple positive-action locks in the ESD installed in the ship make it look complicated and/or expensive, but while it is a very nice solution - very hardware-oriented - it just overloads fusion reactor... CM-88B Bison is a nuclear-powered vessel, and as such the self-destruct, as could be in general in spaceships, doesn't have to be something complicated. Doesn't even have to be a part of the ship's operating procedures - in a pinch it can be "jury-rigged", obviously, because any engineer knows how to intentionally destroy any nuclear device he or she is responsible for, given some time and tools...

In general case of the spaceships there is yet another dimension to all of the above - any space-going vessel can be turned into a kinetic projectile. Or - in words of lucasbachmann in one of the comments to the OP- "Any ship is basically equivalent to a dinosaur killer asteroid".

Though this last one is the only viable reason a cargo vessel would have one, in my opinion, best explanation would be that W-Y Corp. put it in ALL it's vessels in the case of the situation like the one Nostromo encountered. After all, they kind of knew something...