Translation:That is why everything is written together.
It certainly does add an unnecessary ambiguity as now "together" can refer to the outcome of the writing, as in "the words are written together", whereas here, like you, I think the meaning is that the process of writing is carried out jointly/together.
How do you efficiently tell when it is used in a certain way?
Eg. when is it "for that reason, when "about that" (surely not this=here=hier?), when something else (what else?)?
To make the passive in German you use WERDEN where English would use BE. It's as simple as that. So "is" in English becomes "wird" in German with a past participle. It gets more amusing. The past participle of WERDEN is GEWORDEN and is conjugated in perfect with forms of SEIN, but in passive constructions it becomes WORDEN. So "It has been written" comes out "Es ist geschrieben worden."