If you don't know what stablecoins are, read this Wiki article
With that read, there is an ongoing dispute over the advantages of these coins before Bitcoin and other cryptocurrencies. Many suggest that they are not actually backed up by real currencies or other assets as the companies behind them vigorously claim, that they are centralized beyond any hope, and without proper audits they can be printed out of thin air just like fiat money, which would culminate in their hyperinflation, devaluation and failure
For simplicity's sake, let's assume that these accusations are unfounded. So do these coins have any advantages compared to Bitcoin under given assumptions? In these conditions stablecoins essentially become fiat that they are representing, i.e. the US dollar, Euro, or whatever. So the whole thing is not so much about some stablecoin versus Bitcoin but rather a good old question of a fiat currency versus a cryptocurrency
And I guess you already know the answer to that question
Stablecoins are essentially debt instruments that don't give any interest in return.
I wouldn't say that they are useless, they are quite useful as a tool of circumventing restriction to foreign exchange in certain countries with capital control, or to buy/sell cryptocurrencies with without having to go through all the verification needed to deal with actual fiat.
But as a store of value, they are garbage. Even though people may perceive fiat as a good store of value, in the long run, it will depreciate due to the fact that the currency supply is continuously increasing without limitations. And they are risky because you simply do not know if they are printing coins out of thin air, or if they are actually backing it up with reserves.