Q: I have just started learning Polish and I wanted to know what the least used grammatical case of Polish is, just not to focus on less necessary things.
A: In frequency terms, the least used grammatical case is the vocative (“wołacz” in Polish), and it is used when addressing or calling people (e.g. “profesorze!”, “kobieto!”), or when addressing objects or ideas, often in literature (like in poetry, for example: “poezjo!”). The vocative is especially required in written correspondence or official situations, however in everyday spoken language the majority of vocative forms have been replaced by the nominative. There is also another case that is required by a lesser group of verbs and thus less frequently used in common Polish than the other cases, namely the dative (“celownik”). In the majority of basic Polish courses these are presented only in a limited, utility-focused range.