Boosts the rate of critical hits.
Super Luck boosts crit rate by one stage. Pair it with high crit moves like Slash or Night Slash and you're critting pretty often. Honchkrow and Murkrow get this, and its nice on Togekiss too if you get lucky with the hidden ability roll.
Here is a cool note, if your lead has Super Luck as its ability, wild Pokémon will then have 60% chance to hold their common item then 20% for the rare one instead. This is the same effect as Compound Eyes. So if you're hunting for held items, Murkrow is actually a solid choice to lead with (also considering it knows thief, its a great item hunter).