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
Well i can tell you right now another disadvantage they have is that they are tokens using someone else's blockchain. So even if you get a wallet, you still need two different coins in it, the stablecoin itself and the coin of the blockchain it operates with to pay tx fees.
With Tether its more confusing, because there are 3 variations that work on 3 different blockchains: Bitcoin, Ethereum and Tron. The others are all ERC-20 so just Ethereum (gas). You need both coins in your wallet to move a transaction, that is not user friendly.
For the time being they appear to be intended to be used inside an exchange as hedge in trading or such, but not directly like you would use any other crypto...