It can be used in smart contracts, DeFi (such as a decentralized crypto exchange or earning interest), and can be used for very fast transfers between centralized exchanges/services that might not allow actual USD deposits/withdrawals or that require waiting for an ACH transfer to go through. Several cryptocurrencies are good for transferring between centralized services, but USDC will be price stable in comparison. Fees can be a problem though.
The problem is other stable coins are not transparent, and are very likely not fully funded so they can collapse any time. USDC is by coinbase and a little more transparent, thus less likely to collapse in case of mass withdrawal.