How is “days destroyed” different from “amount of BTC” in existence? According to https://en.bitcoin.it/wiki/Bitcoin_Days_Destroyed , if I have 100 BTC for seven days and then spend them, that’s 700 days destroyed.

But if I didn't have them, someone else would have them.  So how does the metric differ from the amount of BTC that exists?
Days destroyed are measured in BTC * days
Amounts of BTC are measured in BTCs

The two concepts are as different as Power (Watts) vs Energy (Joules).