"There is no need for a "why" however, as the why of something is determined by causality itself, which is at the users' command, making it one of the few powers that reasonably don't need a reason." from the page

So they can just do it there's no reason for it they just be able to?.

Yep. they don't need any cause, or a the cause could be something so mundane as breathing or blinking. cause=get hit by or be near something, effect=replicate/mimic it.

Well I guess thank you @Nicholas8293

I would more interpret it as (since it is an application of basic causality manipulation) that when someone or something uses a power/skill, the user can create a new cause/effect relation, copying the observed effect to a cause they can make. From this, they have essentially replicated the power/skill by creating a means with which they can use it.

For example, if the user is facing someone who can direct electricity like lightning, they could see the cause/effect relationship behind that person's power (cause: they use their power, effect: electricity is directed towards a target) and replicate the effect, perhaps something like cause: pointing and saying 'fire', effect: electricity is directed towards a target, and suddenly they have essentially replicated their opponent's power.

Really, there are many ways it could work.

@AzQth I see well thank you for giving your two cents on it.

Are you sure it won't take ages to set the Cause and Effect mid battle? XD

Entirely depends on the user I guess. It could just be a thing of, as @Nicholas8293 suggested, a pre-written cause/effect. For example, 'cause: be in a threatening situation, effect:develop a power to counter the situation', though that particular one would be more suited to Superior Adaptation or something like that.