Quite a logical question. After all, investments are inevitably associated with risk.. Why risk losing money, if you can just save them in a secluded place or buy them something you need? Or at worst put in the bank - there even interest can be received.
With the option to “put money under the pillow”, everything is quite simple - inflation gradually eats it up.. Money depreciates over time, their purchasing power falls. If you focus on the official data of Rosstat, then for a year of "lying under the pillow" of each 100 rubles you will lose 5. And this is a good year without any economic turmoil..
As for currency fluctuations? If the dollar has doubled in price against the ruble in a couple of months, this means, that your ruble savings have become half as much. If the dollar has fallen in price, then your dollar savings have decreased.
Investing makes your money work, and income from them at least offset the impact of inflation. And the income received above inflation forms a reliable cushion in case of currency fluctuations..
There is a similar problem with shopping - things get cheaper over time.. And they are falling in price rapidly. Try to sell a used smartphone or computer for the same price, by which you purchased it.
And finally banks. Firstly, interest rates on deposits are very close to inflation, which already creates serious problems - in a disastrous year for the economy, the rate may well turn out to be lower, and your savings will be depreciated. Secondly, a bank deposit is not at all devoid of risks. The deposit insurance fund appeared for a reason - the risk of, that the bank will burst, and your money will "dissolve in thin air", quite real. That is, in the case of a bank deposit, we also risk, as with investments, but we earn either much less, or nothing at all.
The financial world is fast-paced and ruthless, you need to run very fast in it, to at least just stay put. And only investing gives you the opportunity to do it with confidence..