They can help us establish a date for the eruption
The traditional date for the eruption is August 24 79 AD. We get this from Pliny the Younger who—being the good, proper Pliny that he was—dated his letters “nonum kal. septembres” (“nine days before the first day of September). However other manuscripts, based on an original that’s now lost, date it to November. This inconsistency could be down to mistranscriptions (the everyday bane of a medieval scribe’s life). But, problematically, the later historian Cassius Dio also hints at a November date, writing that the event happened at the end of autumn.
Unfortunately, we’re 2,000 years too late to ask Pliny for clarification (that, and we don’t speak Latin). But what we can do is turn to Pompeii’s silent witnesses to shed light on the issue. Curiously, the plaster casts of the victims show that, at the time of the eruption, Pompeii’s population was layered up in warm clothing. And anyone who’s spent summertime in Central-Southern Italy will tell you that, during the month of August, this would have been completely and utterly insane. Of course, it’s possible that August 79 AD was unseasonably cool, and the fact that archaeologists found numerous used braziers in the House of Menander does seem to suggest this. But there’s more.
In the House of the Golden Bracelet, archaeologists found a coin among a fleeing group of Pompeiians that’s at odds with the chronological context of August 79. The coin was minted in honor of the incumbent emperor Titus and—in typical Roman style—consists of a long and rather tedious list of his acclamations and time in office. The golden giveaway is the inscription IMP XV: the emperor’s 15th imperial acclamation. This is significant because we know from the emperor’s own hand that, even during September 79, he was signing letters still gloating about his 14th imperial acclamation.
Some still cling to an earlier date. Anna Maria Ciarallo, for example, has used the large amounts of garum (a fish sauce used extensively in Roman cooking) found in Pompeii to push for an August date: arguing that there would have been more of it on the market then. On the strength of the evidence, it’s likely that the eruption happened later in the year. What’s certain is that, if this debate is ever settled, the conclusive evidence won’t come from garum or manuscript editions, but from Pompeii’s bodies.