[Jimbee (OP)]

The vocative case in Latin is the noun of direct address. Why does it need it?

According to Shakespeare, Julius Caesar's last words were Et tu, Brute? meaning "Even you Brutus?". Brutus is in the vocative case. But I just said it without using it. And you understood what I meant.

So why did the ancient Latins think they needed this special case?

[Bored chemist]

It doesn't; because it's dead.
On a related note, why did you forget the noun?
Why Does Latin Need A Vocative case?

[evan_au]

A tangent on Latin...
There was a rule in English grammar that you should never split an infinitive eg "to boldly go where no man has gone before"
- This rule for English was apparently derived from Latin
- Apparently, in Latin, it is impossible to split an infinitive, because it is part of the same word (pardon me if I have confused this explanation - I didn't study Latin)

https://historyofenglishpodcast.com/2023/01/30/episode-165-glamorous-grammar/

Let's face it, languages dynamically adapt to the changing situations they find themselves in (until they are dead...), and so their current state is sometimes hard to explain...

[alancalverd]

The Latin vocative is important because word order isn't. English distinguishes between subject and object by the usual transitive order "A did X to B" and you could include an implicit vocative with "John, I think A did X to B". But you don't find much punctuation in written Latin, so you need to identify "(nominative A) (accusative B) (vocative John) cogito X fecit".

It's odd that modern Romance (Latin-based)  languages use transitive word order but formal German, with a very different root and history, often puts the verb at the end.