The first and foremost aim of using stable coins is to maintain stability that will help traders to retain their portfolio value against volatility. Traders, holders and users can know their portfolio value or transfer value with real assets like USD, EURO or Gold. In times of need or when traders or investors think this is time to stop their trade and keep their value before volatility, they can exchange their cryptocurrencies with stable coins. Job holders in cryptocurrency related firm can get their salary on the basis of stablecoins and remittance fighters can use it to send their remittance to their native countries within few minutes.
It is not just "important" but available, that is one of the reasons why it is there. For example if there was nothing called stablecoin, would we live in a different world? Would everything be so much different? I am not so sure, we may have lived the same life right now, what do people use it? To turn it into fiat pegged price without going out right? Well many local exchanges (and some big like coinbase) does that without any stablecoin at all, I could turn my money into fiat on my local exchange wait there and then go back to crypto without ever using crypto, and if I want to transfer it, I could just buy a low fee coin and then move it and then turn into fiat at the place I go to, without needing stable coin at all. So it is not just about being important but about being available, its available and even though it is not a "must" it is definitely used because the option is there.