>>725018850 (OP)
In Binding of Isaac luck increases the chance of collectables to drop, as in coins, keys, bombs, etc; as well as increasing the probability of your arsenal to dish more damage. For example you have a tear effect that burns enemies and a chance to actually fire these tears dependent on your luck number.
In this game the more luck the better, but it depends on how devs balance this stat and the items dependent on it.