I notice that sometimes Shapefiles that I have received contain .prj files. Sometimes they don't. Wikipedia notes that the .prj file is not "mandatory" and that it contains the :
projection format; the coordinate system and projection information, a plain text file describing the projection using well-known text format
So my question is: why is it sometimes included, and sometimes not? Do newer versions of Esri tools include it, whereas older ones didn't?