Then Gideon said to God, "Let not your anger burn against me; let me speak just once more. Please let me test just once more with the fleece. Please let it be dry on the fleece only, and on all the ground let there be dew."
Gideon wanted a sign from God that he would receive victory. He laid a fleece out over night and asked god to put dew on it if they would win. A bowl full of moisture from the dew came out when Gideon squeezed the fleece.
Gideon was not satisfied with this and asked for no dew the next night. God honoured this request also.
It's not clear why God deemed it acceptable for Gideon to question him and be answered in this way. It would seem like a lack of faith to ask once, let alone twice, but God must have had his reasons. Perhaps because Gideon faced quite an extraordinary situation requiring God to build his faith to an extraordinary level. Whether this is true or not is irrelevant since God doesn't need a reason if he wants to do something - he knows exactly what we need to believe. Some may need more than others but it is up to him when he draws a line at providing evidence and from then on we need faith that he will come through as he has said he will.