Stablecoins are cryptocurrencies whose market value is tied up to another currency or to external references. They are more useful than being more volatile cryptocurrencies as means of exchange
It is not so clear what you want to say here, but stable coins might be "stable", but they can be depegged at anytime, stablecoins is no doubt useful in the cryptocurrency industry, particularly for traders, but because of the risk they pose, risk such as centralization and the ability to freeze people's funds, it is not advisable to hodl stablecoins for too long.
It aims at providing and offering a choice to high volatility of most popular cryptocurrencies including Bitcoin.
Stablecoins is not an alternative to Bitcoin, they are very different types of crypto, to name one example, Bitcoin is decentralized, whilst stablecoins are centralized.