Traits and stat ranks create a long-term resource sink. The best choice depends on whether the unit is a DPS carry, support, tank or farming unit.
Damage-focused units value output and range traits, while survival units care more about HP and defense. Economy traits can be valuable earlier in progression.
Ranks range from low tiers up to SS and SSS. Because high ranks have low weights, a reroll calculator should show both expected cost and chance within a fixed number of attempts.