Luck is very useful in Jacksmith because it increases the chance of gaining more green gems (the currency of the game which can be spent in Gander's shop) and boosted weapon parts.
Luck is boosted by building weapons using parts that increase Luck for the individual wielder of the weapon, or by building Epic weapons that increase luck for the entire group of fighters. However, be warned, Luck-boosted Epic Weapons are usually made from Bronze and/or Copper, so they will not last very long.