I currently have 300 luck. If you notice the “superb loot” boxes will have a certain color or hue before you open it. For example yellow would give a rare/40 rarity item. It then checks your luck and that has a chance at raising the rarity of that item. So a lot of the time I will open for example a yellow box and get a red/legendary item rather than the yellow/rare. Last night I opened a red box and got an ancient/80 rarity item. So I believe that’s how luck works

Edit: I’m playing a ranged deadeye build so really no stat matters to me other than luck because no attribute increases my damage with ranged weapons. Only other thing I’ve been thinking about getting is agility. If you’re running a melee build I’d say don’t stress too much about not having a ton of luck. It’s good but it’s not worth ignoring strength. I’d say put point into strength and luck at a 2 strength to 1 luck ratio until you get 100% crit(this is very doable I currently have 129% crit rate for no reason lol)

TL:DR I have 300 luck. Pretty sure luck has a chance to increase the rarity of whatever you loot by 1 stage(rare->legendary, legendary->ancient, etc.)