Most cryptocurrencies swing in price. Stablecoins are designed not to — each coin aims to stay worth about one US dollar.
Traders use them as a safe harbour: when you want to step out of a volatile position without cashing out to a bank, you move into a stablecoin like USDT or USDC.
They're also the base currency for most trading pairs, so understanding them makes the rest of an exchange much easier to navigate.
Stablecoins aren't risk-free — the quality of what backs them matters — but they're the calmest place to sit while you plan your next move.