Passives are force multipliers
A passive item never fires a shot. It changes how your weapons behave: more area, more speed, more projectiles, more survivability.
That makes passives quiet but strong. The right one turns a good weapon into the thing that carries the run.
Which ones earn the slot
Area and projectile passives raise the raw usefulness of most builds. Speed passives help you stay alive when the screen fills.
Defensive passives matter less early, when enemies are weak, and more later, when one hit costs real time.
Slot them around the core
Take passives after your main weapons exist. A passive with no weapon to boost is wasted space.
Pair each passive with the weapon it helps most. The evolution mechanics page explains why leveled weapons beat crowded ones.
Next: How weapon evolutions work.