Well, the official story is that
a) they wanted to humiliate the victims before they gassed them. In a "survivor tesitony", one "survivor" said that "the Germans envied the beauty of the jewish woman, so they shaved their hair".
Jews are known for a lot of things. Beauty isn't a part of it. Imagine Deborah Lippstadt saying that.
but especially:
b) the super-efficiant Germans made mattresses and slipper-soles for submarine crews out of the hair
Apparently there was a shortage of hair. Now why didn't these Germans use their own hair? The hair of their soldiers was pretty long.
It's very important to never do the math yourself. They gassed jews, took children by the feet and smashed them against walls, because they didn't want to waste bullets. 6 million bullets, that's a lot, even if the German pre-war economy produced 723 million 8mm Mauser rounds in the first 3 months of 1939, so 8 million rounds PER DAY before the war had even started.
6 million bullets - enough to give every soldier in an infantry division 350 rounds. It has to be saved. They couldn't just use the captured 42 million norwegian 6,5mm rifle rounds for their Krag–Jørgensen rifles that the Germans didn't really use for logistical reasons, or some old black powder rounds for obsolete revolvers and rifles that you still had all over Europe.
Just believe it. 6 million bullets was worth building these super-efficiant, Nazi-space magic "assembly lines of death", where they also figured out how to cremate a body in 5-7 minutes. Just make a big corpse sandwich out them before you shove them in the oven. Building more ovens would have been too expensive for a nation that spend 2,5 million on a single submarine.
btw: did you know that the Germans used the ashes of dead jews as fertilizer for their Sauerkraut-fields? They used everything. The fat for soap, the hair for slippers and matresses, the skin for lampshades, gloves and knife sheaths, the gold teeth to keep the war economy rolling, and the ashes as fertilizer.
And the hair? Imagine sleeping on a mattress that's just a sack full of human hair. If they thought that's a good idea, they couldn't just collect the hair in the barber shops of the 80 million-country of Germany. Maybe jewish hair is like goose feathers. I dunno. Don't think.
They just did it because (quote Matt Cockerill) "they were fucking barbarians". Now stop it with the "math"-stuff. This is german logic. They do this kind of stuff because that's just how they are. Stop arguing. These facts have already been established by people with credentials.
