what are those tubes on the xenomorphs back even for anyway?

Filtering elements out of the atmosphere to use for mass and...snorkeling.

In the Script for Alien Resurrection the tubes on the Xenomorph's back were used to spit globs of resin that the aliens used in cocooning and making Hives.

Its very open for debate...

Likely reasons would indeed be..

1) Some kind of Breathing/Filtering Apparatus

2) Someway to secret Resign to Build Nests, Cocoon Hosts.

Problems with this are from Aliens and Alien 3.

a) The Queen does not have them, so maybe they are no kind of Breathing/Filtering Function. Yet the Queen was in the same Environment as the Xenomorphs.

b)The Runner from Alien 3 did not have them, and so we could think its because the World it was birthed from had breathable atmosphere?  But then this brings me to the first Point A)

Do we have to ask what the purpose of the Organism was?

It appears to kill and Procreate, and so then if this is what the Drone in Alien was doing, and if the Tubes are needed to create a Nest/Cocoon Hosts then we have to ask would this not be the purpose of the Runner... to Procreate?  And so why does the Runner not have these if these are used to Cocoon Hosts etc?

A thing to consider is Ripley is carrying a Queen and so maybe Pheromones and what not that affected the other Face Hugger and what kind of Organism it Produced,  getting some kind of Signals that a Queen is hosted inside another Organism, maybe the other Face Hugger did not require a Drone like in Alien?

But then if the Tubes are effected by a Queen, then why did the Drone in Alien have them...

So yeah.. really Good Question, and one that we can have some ideas but then when we look at the Runner and Queen it raises some Questions.

I would say i will have to Pass no this one ;)

SM no doubt would have been better to ask ;)

If you look closely on the engineers suit when he gets out of his crypod when he gets woken by david and company, there are 4 round holes on the back of his suit positioned roughly where those tubes are located, maybe they are mutations of those holes?, ridley said they are organic suits - just speculating, but those tubes could be anything.

The Engineers suits always remind me of the Centurions Cartoon

Where their suits are Bio-Mechanical Versions of these, that allow the Engineers to connect with their Technology/Ships

I see like 10 such connection holes on the Engineers suits, 6 on the lower back area, and two on the sides and two in the lower midsection.

@BigDave, its just another mystery to ponder the rest of our days lol!

I read somewhere they were there to make the xeno look more alien and less like a guy in a suit.

Seriously, they are for snorkeling.

dk I remember something about that too. They didn't want the silhouette to look too humanoid, so when you see glimpses of it in ALIEN you know its not human. The tubes break up the human shape.

I always assumed it was some type of sensory organ for vibrations or "micro changes in air density" ;p

@nightstar81 Gave the answer i've always understood, that they were used to secrete the resin that made the hive walls/coocoons. Like a spider's spinnerets.

I think they would need to be on the front if they were going to secrete resin.

I think its one of those things that hopefully we get a explanation, but i agree with the comments that ultimately it was a design Atheistic to make the Alien look well Alien, i am not sure they ever thought about the purpose of the Tubes when they made Alien.

So its fun to speculate what their purpose is for.

@Kethol; I'll have to find the text where it explains it but the Alien is basically upside down while doing it if I remember correctly.

Giger says they're for balancing their elongated heads: Dorsal Tubes

I suppose Svanya thats a good explanation as the Queen would be able to use  her longer Tail for Balance and maybe her head is more aero-dynamic in that sense, then we have those spines.

The runner as it walked on all fours and had a slightly different stance would maybe not need them for Balancing.

Seems the Ret-conned design that was explored with Alien R where the Xenomorph no longer has Human like Legs, and how they now appear to move on all fours could change that idea though.

But if it came from Gigers Mouth and its also a plausible explanation, i would be glad to accept that.

Their main purpose is to look horrific!

it's fun to speculate, though. Many of Giger's paintings feature horrible or erotic subjects in a state of entrapment or bondage: terrifying yet entombed or massively constrained. The tubes fit into this scheme of things rather well.