The stone was a part of a steele that was displayed in a temple.

The translated text is essentially an announcement or decree for a new Egyptian regime, and wasn't necessarily written with the intent to preserve history.

The reason why it's in three languages is because each of those languages served a different purpose.

"hieroglyphs (suitable for a priestly decree), Demotic (the cursive Egyptian script used for daily purposes, meaning 'language of the people'), and Ancient Greek (the language of the administration – the rulers of Egypt at this point were Greco-Macedonian after Alexander the Great's conquest."

You can read more about it here or do your own research. https://www.britishmuseum.org/blog/everything-you-ever-wanted-know-about-rosetta-stone

AfaIk, the Rosetta stone contains a Ptolemic decree which was, to be broadly understood, usually published in all three official languages / writings in Ptolemic Egypt, i.e. Greek and ancient Egyptian in Hieroglyphs and Demotic writing.

I mean, stone carvings are always meant to preserve history. That's why they're in stone.

Nothing quite so explicit as that I think, though obviously preserving something is always the intent when carving shit into stone.

Ptolemaic Egypt was a culture populated by Greek and Egyptian speakers. Of those who were literate, many would only be able to read Demotic or Greek, but meanwhile there is a 2500 year history (at THAT time. Egypt is ridiculously old) of Hieroglyphics being the "official" way to write things down, and the scribal and priestly classes would be part of the cultural elites. Combine that with the Ptolemies attempting to situate themselves as both continuing Alexander's legacy and being fully Egyptian, and there will be a place for all three scripts. Engraving laws onto stones and placing them in prominent public spaces would have been a pretty common way to "publish" them in a way that's meant to be durable and secure. See the Code of Hammurabi and Draco's code for just a couple of examples.

The fact that Alexandria was cosmopolitan and had a sophisticated regime ruled by elites who were foreign but invested in the local traditions created a situation where this was done often enough for some to survive, several in fact, although the Rosetta Stone was the first found/identified.