Maybe it is to show that the field has been edited at some point? 🤷🏻‍♂️

Zero Length

NULL v. Zero Length String

Thanks Guys.  HI Alex:  I found the NULL vs Zero Length String link after posting.  Richard's statement (below), while accurate and valid, is extremely subtle.  
I am left feeling this is an artifact of how the programming language initializes variables.

excerpt From NULL vs Zero Length String:
Let's say you're tracking first, middle, and last names, and you NEED to know whether or not someone has a middle name. You could set their middle name equal to a ZLS to indicate that they have NO middle name, as opposed to not knowing what their middle name is. (NULL implied here..dmh)

A zero-length string is considered a string, and null is not.  You can't use null when a string is needed.  They may look the same or mean the same for all practical purposes.  But in coding, you need to use the correct data type, one that is required by the code.  For instance, the function Left(s, n) won't work if s is null and not a string.