Flash loans are useful for both arbitrage and loan liquidations. Every swap on Uniswap and its many forks on many chains is actually a "flash swap" under the hood - you can take the coins out and use them before the code checks that you have sent the tokens for the swap in to the contract. You don't even need a dedicated flash loan provider on a given chain to be able to use very large amounts of capital sitting in DEX pair contracts.

There are other case-specific uses for them, but loan liquidations and arbs are the big ones.