First and above all the apostrophe stands for a dropped character, i.e., an elision. When its (sic!) missing it's even more annoying than when there is one too many.
In the "Cashier Talking Points" below you will find two "grocer's missing apostrophes" where according to the blog's author the second one is a twofold blunder with "Pretzel Crisps" being plural: Promote that they are on sale.
What about the apostrophe of the English Possessive Case (Sächsischer Genitiv/Saxon genitive)? Also here the apostrophe stands for an elision. In old English the genitive of dæg (day) was dæges that was slurred to day's: It's a hard day's night.
In modern English the possessive is particularly used in case of persons: Walter's car, Jesus' or Jesus's words. This is what Red Baron was taught. Constructions such as Freiburg's University were not promoted in my English school grammar of 1946 although now are perfectly at ease in my modern English grammar of 1964.
Since 1901 the German language has no longer supported a Possessive Case. However look at all four Wurststands on Freiburg's Münsterplatz more than one century later:
|No teacher taught them the Deppenapostroph|
|The Nec plus ultra (©Wikipedia)|