Stable coins aren't investments, I think it's better to say that use stable coins during the bull run so that you'll keep your profit. Investing on it means that you're going to treat it as an actual investment even it's not.
Its value isn't moving so there's no sense if someone says that a person should invest in stable coins. That's just a little correction and you're right that it's like putting your money in the bank.
People who think that they should "invest" into stablecoins, do not understand the whole point of stablecoins. USDT type of stuff are there so that you could turn your crypto into fiat, without actually going into fiat. Back in the day people had to use companies and exchanges that work with fiat, and places like Binance, or cryptsy, or poloniex or bittrex etc etc did not have that fiat option.
So, you would use local exchanges. What you did was put your money from your bank account to your local exchange, turn it into some altcoin that would be cheap to send, send it to poloniex, trade there however you want and when you want to go back to fiat, you had to repeat that in reverse. Stablecoins prevented that and you can now go to stablecoins directly. That's all they are good for, nothing else.