1 word. Inflation. Always invest when you can as the value of your money will always diminish over time.
Thats will only happen if the money that you save is paper money. If you save bitcoin or gold you will not losing your value of your money. Bitcoin is good as investment and good as money ( currency )
One can always spend the money and gain bitcoin or altcoin in that exchange rate. You dont have to rely on paper fiat currency always. But still its more convenient to keep some paper cash handy otherwise you might run out of money at the point when you might need it. Bitcoin since its starting has been a good investment as a trading commodity. People buy it because of the price and sell it when prices rise.