Wrong framing, currencies don't have agency. You should be asking when would you need your currency to be devalued, regardless of what it's called or made from.

And the answer to that is all the reasons governments do just that, except for the times where the government is being particularly stupid and doing hyperinflation.

When the economy grows, the amount of currency needs to grow as well. Otherwise prices will fall (deflation). That hurts the economy as a whole because e.g. real wages might increase too much