Why did Gideon use a fleece and the action of the dew as a sign?
Genesis 27:39
And Isaac his father answered and said unto him, Behold, thy dwelling shall be far from the fatness of the earth, and the the dew of heaven that cometh from above;
In Genesis 27:39, Isaac speaks to Esau after being deceived by Jacob, who received the blessing meant for the firstborn. This verse is part of the "blessing" Isaac gives Esau under distress and sorrow. The phrase "far from the fatness of the earth" suggests that Esau's life will be marked by hardship and scarcity. In contrast to the abundant blessing given to Jacob, Esau is destined for a more barren and challenging existence.
Gideon wanted to be sure of an easy victory over the Midianites, but God makes things difficult in chapter 7.