Why does Weyland Yutani want a xeno for weapon when they have, for example, androids?

Question

I could see xenos being useful from a medical standpoint but in a time when technology includes not just FTL drives but also controllable androids, what could xenos do as a weapon that would be useful?

Answer 1

For the original movies, the company doesn't know precisely what the Aliens are all about. But they know enough to casually risk the lives of innocents, in the hopes that they will obtain some useful advantage.

Alien: Special Order 937 told Ash to "investigate a life form, possibly hostile and bring it back for observation." Maybe they haven't told Ash everything they know, but the sense is that they've underestimated the dangers. It would be ludicrous to send an uninformed freighter crew to pick up a xenomorph otherwise. And note that Ash is not trying to kill everybody, he just wants to bring the alien back, and the crew is regarded as expendable - even when W-Y doesn't know what a prize the alien really is.

Aliens: Years later, and as a result of learning about the events of the first film, the company knows a lot more about the xenomorphs, and they've positively decided that retrieving them would be worth "millions" to their bio-weapons division. This justifies (in their eyes) the sacrifice of random civilians and a military team. Once again, though, they've underestimated the dangers present - though if anything that makes the xenomorphs seem even more useful!

At this point, the company knows that a single xenomorph has absurd capabilities in hand-to-hand combat and survival. The infestation from a small number of eggs was bad enough that (the survivors of) an elite squad of Colonial Marines wanted to nuke the site from orbit as their best option. This suggests that even as-is, the aliens are a terror weapon that outmatch conventional military capabilities. And although some of the subsequent media in the franchise has military androids, what we see of Ash and Bishop does not seem to put them on the same danger level as the xenomorphs; Bishop has some knife skills but the alien can rip him apart.

But ultimately, Weyland-Yutani is the kind of company that is quite happy to throw away other people's lives for the sake of profit, or even a chance at profit. It doesn't need to be the case that they have carefully assessed how to integrate xenomorph-derived knowledge into their technology, or carried out an analysis of android vs. alien capabilities. They saw a chance and they went for it.

In Alien Resurrection we do see the kind of thing they were able to accomplish, aiming to create a super-soldier. And in other subsequent media, not just films, we see a lot of variations of aliens and androids, and get to learn more about them. This is how expanded media ends up. But Weyland-Yutani's motivations make sense on the level of the original story, where we don't have all of those details but do understand that W-Y are assholes.

Answer 2

Xenomorphs can replicate themselves, quickly and without extra effort

Xenomorphs can replicate themselves very fast and without any supply or effort (unless you count the effort spent on eliminating their victims). Androids might be able to produce more androids using resources of the enemy (if the necessary high-tech parts are available), but the time and effort spent on replicating is time and effort lost for the main objective. Also, creating an android likely takes more time than a xenomorph needs to fully grow -- and by then the xenomorph has already killed several victims.

The advantage of androids is that they are able to be given other objectives than "kill and multiply" and that they can adapt their objectives to the circumstances, ask for new objectives, or simply give up an impossible mission. So if you want controlled destruction and goal-oriented problem solving, an android is probably the better choice. But if you want to wreak havoc, send a xenomorph.

There are other weapons of mass destruction, but at least in comparison to known and existing examples, xenomorph again have qualities that may make them the preferable choice. In particular, none of the existing nuclear, chemical, or biological weapons will actively hunt its prey, while xenomorphs do exactly that. Even protective gear will succumb, if not to the raw violence, then to the notorious acid.

Also, creating and deploying the currently known weapons of mass destruction can become costly and time-consuming, if a larger area is to be affected, whereas xenomorphs spread out and deploy themselves. Current biological weapons can also propagate, but they need a host for that. Propagation by wind or water dilutes microorganisms, so isolation works against them -- against xenomorphs, not so much.

Finally, cleaning up after widespread use of weapons of mass destruction is impossible, or at least not sensible. Removing xenomorphs after usage seems doable, as long as no new prey becomes available (e.g. using androids or remotely controlled drones). Moreover, if a xenomorph-specific pathogen can be found, cleaning up xenomorphs is a piece of cake compared to cleaning up nuclear, chemical, or microbial contamination.