I wish I knew…and this is a bigger question than you think you’re asking. When we count we go 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and then we start over again, just changing the second number, 11, 12, 13…etc. This is called “base 10”. The base is the number that you have to hit before moving a decimal place over. We use base ten, presumably, 100% because we have ten fingers.
However, 12 is possibly a better choice. Ten is only divisible by 1, 2, 5, and 10 while 12 is divisible by 1, 2, 3, 4, 6, and 12. So for a lot of applications, base 12 is easier to use. And we do use base 12, just not very often or very precisely. Every time you say “two dozen” you’re using base 12. Or, in the case of your pack of tampons, eight dozen.
Why we use dozens isn’t exactly clear…it may be just because it’s mathematically convenient…or it may be good for marketing reasons (96 might sound more impressive than 100.) Or maybe it’s because there are roughly 12 lunar cycles per year (which is where we get the 12 months.)
We don’t really know…but beer, soda, eggs, and tampons…all come in dozens…for reasons that stretch back, possibly, to the very beginning of counting. Which is REALLY COOL.
Using dozens is actually a product of Sexagesimal number systems used in ancient Babylon, Sumeria, and Mesopotamia. This system was created for the same reason Hank described, because it made division much easier. This system was created to two reasons- first for commercial use (where the number 12 was more common), and second for use in astronomy.
What is interesting is that we continue to use this system of counting despite having primarily adopted a base 10 system. And that we continue to use this system for what it was originally intended- note that our time counting is base 60 which has roots in astronomy. This is largely because we developed astronomy from these ancient cultures so we retained the system, and also due to the prevalence of these societies in trading, which meant that the dozens system has also remained.
This by the way is what you learn when you are a law student who has to fulfill a science requirement and takes Ancient Science and Technology. Also why a well-rounded education is so important because it illustrates that there are connections between every subject.