Is flour still good after 300 years? or has it totally rotted away? Can I really eat bread made from flour that was ground in 1713? That sounds awesome.
PCs are always adventuring in abandoned buildings, and so it's useful to know stuff like the time-to-live of various household goods.
Of course, a lot of this is for the amusement of the DM. Most of the time, no one will blink an eye if you have copper-colored copper in the 4000-year-old tomb. There's always that one player out of a thousand, though, who will try to draw reasonable conclusions from environmental cues. "Hey, if no one has been in here since the last millenium, why are torches burning?"
While we're on the subject of traces of the past, here's one of my favorite area descriptions from the game module:
33. Kiri Valley
The forest darkens and thickens beside an ancient trail. A cold, dry stillness hovers in the air, and the trees are knotted and bent. Everything seems to watch you. An evil wizard died here long ago. Only his essence remains.
I don't know how long essence remains vis a vis flour - I'm guessing longer. I like how this area isn't important to the plot, but we still get a little throwaway hook to hang DM creativity on. What exactly is the wizard's essence? It might just manifest as a cold, dry stillness, or it might manifest as a ranting ghost. The detail is there for the DM to expand or ignore.