Traits are, well, traits of personality and appearance intended to describe the character in outline according to the source, to be flags for events and to change initial stats.
Strictly speaking, we don't have enough traits to make every single character truly unique, but when we'll have BE with skills, spells, etc., things will improve hopefully.
As example, Not Human is a general trait for all elves, fairies and so on; it has various bonuses and penalties to stats and it could be important for some customers. Innate elven low light vision feat from d&d could be useful in some dungeons only, druid's venom immunity feat could be useful against venomous mobs only, etc.