They were designed to help identify the approximate manufacturing date of the guitar.
For example: a guitar whose serial number begins with the letter and number "E8", would have "theoretically" been manufactured in 1988 (E = in the Eighties, 8 = the 8th year...
All this is public information, avaiable on the web.
I encourage you to take initiative and verify this (and ALL) information independently, for yourself.
The Fender Contemporary Stratocaster and Telecaster models were part of the Fender Japan E series model range.
The previous Fender Japan models exported from Japan were all Fender Squier models.
The neck date simply refers to the date that the individual component was produced.
Given the modular nature of Fender's production techniques, an individual neck may have been produced in a given year, placed in the manufacturing warehouse and remained in stock for a period of time, and then subsequently paired with a body to create a complete guitar in the following year.
There were periods of time when this was not consistently done, (between 19), and there are certainly other examples of short periods of time, and individual pieces, where the dating was simply omitted.
While this neck dating is useful in roughly determining the age of a guitar, it is certainly not definitive.