Think of luck like you would reforging. There needs to be a balance in order to get what you want...
The more luck, the more powerful the items you get, which mean more mods and higher intensities. However with more mods means potentially more wasted mods and/or unnecessary mods. Again, kind of like reforging minus the brittle sometimes. There is a balance to it depending on what you want, and honestly there is no way for someone to really tell you "just use this much luck to faceroll lootchecks". You have to know what you want to work towards by knowing what is available to drop on an item and work your way up to that point from 0 luck in order to find that sweet spot where you're not getting so many wasted mods on items or too few/weak mods on items.
