When God is using us for His good, He often gives us tasks to do that are simply acts of obedience. We think we should be doing really big things and rebel when asked to do something simple. God could have instructed Gideon to build the altar and do the sacrifice without tearing down the Baal and the Asherah pole. To risk something (time and energy ) in obedience before doing anything else is one of God's lessons to us. God also gives us leeway in the way we obey, thus allowing Gideon to do his task at night, with the least risk to his physical self. Gideon knew the situation and probably hoped to salvage some relationship with his father. He needed time to get the altar built and knew that there would be opposition if it was done in the daylight.
Obedience is a sign of faith, Gideon, even though he doubted his own abilities, had faith that with God he could accomplish his task, even though the outcome was unknown.