Answer:
It conveys the disastrous nature of the event.
Step-by-step explanation:
When describing the fires as an inferno, the author is saying that they were absolutely devastating. There were fifty fires spread across the city, which leads the reader to understand that they were probably out of control.
The sweltering heat emanating from the flames and the nights lit orange by the blaze probably made the city feel like hell, especially when the fires finally burned out and San Francisco was reduced to ashes.