The purpose of y.curve.fi is to further increase rewards on iearn.finance tokens.
After you deposit $DAI into iearn.finance you receive $yDAI. You can provide $yDAI as liquidity to y.curve.fi.
Now you can earn trade fees (0.04% per trade) on top of yield rewards
Simply go to iearn.finance/zap and deposit $DAI, $USDC, $USDT, and/or $TUSD and start earning interest and fees on your tokens.