Some modifiers can only be found on looted/bought weapons... other modifiers are exclusive to crafted items. The player then has to decide which modifiers are more important for their particular playstyle.
But if you are playing properly blind first time through you can't decide that because you don't know what the unique item modifiers are and you don't know exactly what the crafting system will do for you either. It's just blind luck unless you look it up in advance.
