My testing indicates that drop bonuses increase the likelihood of additional chests dropping. Each chest rarity has a minimum and maximum number of chests possible. Drop bonuses increase the chances that you'll get closer to the maximum rather than the minimum. Due to insufficient data, it's not clear if platinum drop rate is affected, but there's a very clear effect on number of bronze chests. If you want to farm Advents efficiently and can do so easily on auto, it's definitely worth having a team equipped with +drop rate accessories.