It's the same reason you have money in a (zero interest) checking account rather than holding cash. For some that's the idea of security, others the convenience of spending "digital money" (e.g. wanting to do block-chainy things without risking the currency fluctuation of ETH or other coins).

Easy to send and trade a stablecoin. May also avoid tax in certain countries.

Particularly useful in decentralized finance as it's on the same chain as the token you are trading.