There are multiple cases of epigraphy in multiple languages, exemplified within the Persian empire, e.g. Darius I, where inscriptions were written in multiple languages due to multiple types of people living in a singe area. In Egypt, particularly after the conquest of Alexander, there was a significant population of Greeks, so if an inscription was to be read by everyone it had to be written in multiple languages.

Simply the fact that the diversity in population forced inscriptions to be written in multiple languages so the majority could read it.

The Rosetta stone is a coronation decree, of King Ptolemy V of Egypt. It goes into a bunch of rigmarole about the King's divinity and awesomeness, the people in his court, recent major events of note, and yadda yadda yadda. The same sort of thing that was commonplace for other rulers of Egypt at the time. The Ptolemaic dynasty is well known for such decrees. Such decrees were inscribed in the relevant languages: Greek because the Ptolemy's were greek and there were many people of greek ancestry living in Egypt at the time, and Demotic as well as Heiroglyphics.

The reason the Rosetta stone is considered special is that it was the first example of such a decree discovered and the first instance of the same text transcribed in Heiroglyphics along with other languages, making it possible to get a handle on the then unknown Heiroglyphic language. However, it wasn't the only one, something like 8 other similar stele and inscriptions of Ptolemaic decrees have been found since (mostly in the 19th century).

What aspect of it are you inquiring about?