"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.