Pharaoh wouldn't let the Hebrews go - not out of love for God or the Hebrews. Not from guilt or respect or fear or reason or wisdom. No, it took force. And how did God know that all non-forceful ways would not lead Pharaoh to let the Hebrews go? Because He knows what anyone would choose under any circumstances. This passages support for middle knowledge is less famous then say, Matthew 11:21, but it is more broad. God knew that a vast array of options Moses could have tried would not lead Pharaoh to freely release the Hebrews.