weapon description

Some weapons have descriptions that say "lengthens X duration on foes by Y%) where 'X' is the status type (poison, crippled, etc).

My question is, do these weapons 'cause' the status type? or only lengthens it if the foe already has that status?

It only lengthens it once it has been put on them.

It does neither, actually. It lengthens any conditions that you apply while wielding the weapon, no matter how you apply it. Attacking doesn't affect any conditions already on the target. If, for instance, you use technobabble or fevered dreams or something like that while wielding a silencing weapon, the dazes will be longer, even if you never attack once with the weapon.

Quick adendum: afaik, the condition lengthening ALSO applies to conditions that you apply to yourself (for example, the necromancer skill "Poisoned Heart").

A quick clarification: It only lengthens conditions that you apply, even if someone or something else causes the same condition.

Imho - in PvE, except for Silencing, most condition lengthening mods are overall less effective than direct damage mods like Vampiric or Sundering. Most often, if things are going right, the foe will die before the condition runs out, or the condition will be removed, or you'll have an easy opportunity to re-apply it. Meanwhile, you could be doing extra damage right now by using Sundering or Vampiric.
Lengthening Silencing is more effective because Daze is usually relatively short, and it has the benefit of reducing the target's ability to do damage.