Why a "dozen"?

[mangeorge]

I boughy a dozen tamales from my neighbor last night, and got to wondering.
Why do we put (mostly) foodstuffs in units of a dozen? And do other countries do the same.
Conspiracy theory: It makes it more difficult to figure the price each.
Peace,
mangeorge

[Max_Torque]

Well, I think at least part of the answer comes from this old Straight Dope column on the subject of why hot dogs come in packages of ten, but buns come in packages of eight:

The true explanation, in my opinion, is that bakers just don’t like tens. They prefer dozens, or more generally, multiples of three and four, notably four, six, eight, and twelve. These quantities lend themselves to compact packaging–three rows of four, two rows of three, two slabs of two by two (e.g., hamburger buns), and so on. Ten lends itself only to one row of ten or two rows of five, which are seldom compact shapes. Therefore, the baking mind-set–and here’s where we start getting into anthropology–is such that you instinctively regard ten as an unwieldy number. When the pioneers of bun baking were trying to figure out how to package their product, they probably figured what the hey, eight makes a squarish package, so that’s what we’ll go with, without even considering the unique circumstances that made ten more appropriate.

(bolding mine)

So it’s for the sake of packaging and appearance. Or, to put it another way, marketing.

[Philster]

12 fit in/on the pan and oven

[Gary_T]

Dozens go back to long before packaging and marketing were a concern. The appeal is that it’s easy to divvy up a dozen things among 2, 3, 4, or 6 people. If you ten of them, it’s only easy to split them among 2 or 5 people.

[Gary_T]

Uh, if you get ten of them.

[Exapno_Mapcase]

Things grouped in twelve:

Months of the years!

The OED traces the word dozen back to c1300, so as Gary T says, it was in use way before packaging.

The easy division of the number makes it useful in many professions. And remember that the decimal system, even the use of zero, is a fairly late occurrence in Europe. People would surely use base 10 because of the number of fingers we have, but being able to parcel out 1/2, 1/3, 1/4, and 1/6 of a number of things would be incredibly useful in a semi-literate society. We see this in English money, where the pound was 20 shillings of 12 pence each. And few people would ever have much more than a shilling to deal with.

Twelve is also a divisor of 60, and that makes it part of the sexagesimal or base 60 system developed by the Babylonians. We still use 360 degrees (closely corresponding to the 365 days of the year) and 60 seconds to a minute and 60 minutes to an hour in both time and angular measurement. The Babylonians brought this down to a duodecimal or base 12 counting system.

Add this to the nice coincidence that there are about 12 moons to a year, regularized to exactly 12 months in a year, and two times twelve hours to a day and a dozen becomes the natural way of thinking about groups.

Hundreds of years of traditional use have pounded dozen into our culture. Besides that, the reasons it was useful originally are still true. Add packaging in, and it’s double plus good.