Hiding money under your mattress seems safe. But inflation steals it slowly. It's slow but effective.
The simple example
You hide 10,000 euros under your mattress. Inflation is 2% yearly (normal).
After 10 years, that 10,000 euros buys what 8,200 euros used to buy.
You lost 1,800 euros without anyone robbing you. Time just passed.
After 30 years
10,000 euros becomes the buying power of 5,400 euros.
You lost 4,600 euros.
In a bank (without investing)
If you put money in a bank earning 0.1% yearly interest, you don't beat inflation.
After 30 years, inflation takes 4,600 euros.
In a pension plan (invested)
You put 10,000 euros. It grows at 5% yearly (above inflation).
After 30 years, you have 43,000 euros (in today's buying power).
You earned 33,000 euros.
The lesson
Inflation is slow theft. It does nothing if you do nothing.
Investing (even in a safe fund) beats inflation.
For small savings
If you're saving 500 euros for an emergency coming soon, under the mattress is fine (short-term).
If you're saving 500 euros for retirement, invest it (long-term).
The updated grandma's advice
"Don't hide money under your mattress, invest it" is the new grandma advice.