I've been reading the active item qualities, and most of them say "If you hit you may spend so and so adv to do something." Sunder however just says "spend 1 adv to sunder".
My friends say you need to hit to sunder, but what's the point of sundering if you smack 'em with a 9 base damage lightsaber and then oh look he's dead now I'll sunder his weapon.
Do sunder and knockdown require a hit?
In the beginning of the Item Qualities section of Chapter 5 of the core books it notes that unless stated in the description of a specific quality, a successful attack must be made to activate a weapon quality, but it does not require that damage be done. Despite the high damage capacity of a lightsaber, there are still situations where it may not get through soak, armor, or other considerations.
As weapon qualities are typically the result of their attacks, a successful hit is required. Some, like Guided, can only be activated on a miss and are related to the specific nature of the weapon's attack. Simpler situations like Sunder, Knockdown, or Linked are directly related to succeeding in having the attack connect with the target.
It may cause some confusion when reading that some Qualities state on a hit and some don't, but as this inclusion or its absence does not contradict the text at the start of this section (on a successful hit...) the group can still follow the instructions and description as written. When things are different they are spelled out in the description of the Quality. This might appear as "...on a missed attack..." or something similar.
Sunder, on a successful attack, will damage a weapon or item by 1 step per 1 Advantage spent. Multiple activations are possible if enough Advantages are rolled, each damaging the item one step.
Sunder could be activated through the use of its listed cost of 1 Advantage, or the use of 1 Triumph.
The reasons for using Sunder during the resolution of a successful attack are many, connected to the specific scene, and to the players' points of view and culture of play. Some reasons might be to reduce armor or other defenses to enable more successful attacks later, to destroy a dangerous weapon to allow a different approach to dealing with its wielder, to reduce the threat from an opponent, to encourage a surrender, and other motivations tied to roleplay.
Added Effect
It should be noted that this is not an either Damage or Sunder choice. If you note the sequence of attacks, damage is resolved in Step 3 and Qualities in Step 4. Unless otherwise noted in the resolution of an Item or Weapon Quality (such as Stun) in Step 4, damage will be applied and in many cases lead to the narrative effects of the Quality.
In other words, on a successful hit, but regardless of whether damage is inflicted past soak, the attacker may add the effect of Sunder to the narration for the cost of 1 Advantage, and thereby choose to apply Item Damage to something openly used by the target. Four such applications in a single turn will destroy that item in a single hit. This includes things like armor.
Activating Sunder and/or Criticals
If trying to understand the benefit between choosing a Critical or choosing Sunder, the limitation of inflicting damage becomes relevant. To inflict a Critical requires that damage must get past Soak. Sunder only requires a successful hit. As some lightsabers can inflict a Critical for a single Advantage, this is can become a consideration in play.
Also, the needs of the scene may urge the character to seek avoid grievously wounding the target.
In few words, yes, you need to hit to sunder or use knockdown.
Sundering with a weapon is different than just attacking the weapon (which you can do). By sundering, your PC is converting the momentum from the successful attack and directing it to harm the weapon or other item of an opponent. You don't have to deal damage (pertinent if you face a cortosis-laden enemy), but the attack must be close enough to strike the item.
My friends say you need to hit to sunder, but what's the point of sundering if you smack 'em with a 9 base damage lightsaber and then oh look he's dead now I'll sunder his weapon.
You are right that sundering is not particularly useful against minions, and even many rivals in this system. If you manage to hit the minions with a lightsaber, they usually die (excepting the unlikely cortosis armor), while rivals can tank a few hits but not enough to justify using sunder. Additionally, if you are trying to defeat an enemy non-lethally, you would generally attack the item rather than sundering, as the attack certainly is lethal. The real use of sunder, however, lies in nemesis-level engagements. At that level, especially in Force and Destiny, enemies may very well have cortosis-woven armor and high soak, meaning even a lightsaber could be dealing low damage.
As an example, I will use a standard Inquisitor from pages 419-21. Even without cortosis, the Parry 4 talent reduces the damage of your lightsaber to base 3, against an enemy with a minimum of 22 wounds. In a case like that, sunder might prove to be very useful. If the enemy uses a lightsaber (which is immune to sunder, p. 175 of the core rulebook), you can still make the argument to your particular Galaxy/Game Master that you would be disarming the opponent instead (which might require several advantage). The usefulness of sunder in that case comes from reducing the Inquisitor's damage potential, as well as removing his access to the Parry talent if your GM agrees.
Finally, remember that lightsabers are not the only weapons with sunder. Vibro-ax wielders may commonly find that their attacks are not capable of inflicting damage upon a well-armored defender, but the sundering property still functions regardless of damage.
In summary, sunder may not be useful for lightsaber wielders in most encounters, but it can be a lifesaver against the most difficult opponents.
