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.
You do not make use of stable coins during bull run, rather you make use of stable coins when the market is bearish, you can convert to this stable coins and use them to save your value, instead of withdrawing to Fiat. Stable coins have the same value with Fiat because they are pegged to it, so it is basically the same thing that you are holding, just that this time around it is in digital form.
And it is also very much easy to switch to stable coins on an exchange without having to withdraw and paying all those fees to move it to Fiat. And once the crypto currency of your choice starts to do well you can also quickly convert it to that cryptocurrency right on the exchange. So stable coins like you have said are not really an investment.