I don't know why but I've always had a soft spot for the vocative. I wish English still had it. What is it? Basically a special form of a name you use when addressing someone or something.
A really nice and surprising example, if you didn't know about it, is Scottish Gaelic Seumas, 'James', which becomes the vocative A Sheumais when you directly address someone of that name. This is pronounced uh-haymish and now you know where the name Hamish comes from.
I don't know any Gaelic but I know some Greek. In Greek, when speaking directly to someone you use the vocative. For some names the end of the name may change, for example if you address Kostas it will be as Kosta, Yiannis as Yianni. Other names may not change their spelling but are still regarded as vocatives; for example Maria or Anna, and I think that speakers, even non-native ones, sense them as vocatives
This can be confusing to people learning Greek, who when hearing someone address Kostas, using the vocative Kosta of course, tend to assume he is called Kosta when he's actually called Kostas (hope you're keeping up!) Even more confusing, if you speak about Kostas, you must refer to him as 'The Kostas', Ο Κοστας.
The vocative is also used when addressing someone by a title-based form. For example 'Doctor' in Greek is Yiatros but when I address the doctor I have to say Yiatre. I could even speak to my dog (skylos) in the vocative, Kaló skyle!, 'Good dog'.
Originally all the branches of the large Indo-European family used word endings to mark what role a noun played in a sentence, for example being the object of an action ('I patted the dog')'or being a possessor ('the dog's dinner) or a recipient ('I gave the dog a bone'). There were eight different 'cases' altogether, including our vocative.
Over time some branches of the Indo European family have eroded or abolished the case system in favour of things like word order, and nowadays cases have largely disappeared from all the Germanic languages bar Icelandic and German itself, and all the Romance languages bar Romanian. This included the poor vocative, now only hanging on in Icelandic and Romanian respectively.
But it's alive and well in Greek, as we have seen and in many other branches as well. An interesting exception is Russian, which unlike most Slavic languages lacks a vocative. It was abolished by the Russian government in 1918 on the grounds that it was archaic and little used. However, and this is fascinating, an informal 'neo-vocative' has apparently emerged in Modern Russian. for example Sasha would become Sash.