I just finished writing notes for an essay, and though "if I were in ancient Greece, and someone centuries into the future saw my notes, how'd they know I was really bad a spelling?"
I'd imagine it would be important for discovering an ancient language.
To add to this, what about terrible handwriting that only the person writing it can reliably decipher (like *non-standard cursive-type writing)?
You'd know by comparing it to a more standard text. In 1000 years you could know if any arbitrary text is spelled according to prevailing standards or not by comparing it to a preserved dictionary. We can compare a particular text to the mass of texts to see if a particular spelling is divergent or not.
For languages we already know, we have a decent handle on vocabulary in writing, so if something is similar but different we can guess at what it was supposed to say. This is common in biblical manuscripts, we know what a text says in other sources, and what is a comprehensible reading, so we can figure out if a scribal error is present. One issue is that there are, for many languages, divergent spelling standards. What would be a normal spelling in, say, the Dead Sea Scrolls would be extremely unusual in a 6th century BCE inscription. Neither is a misspelling, both scribes were simply following the norms of their day.