Plagiarism destroys your reputation, the penalty is an immediate ban of the user. IMHO many newbies tend to plagiarise because they don't read the rules and guidelines.

No idea is completely new, only evolving.
Many writers get ideas from other writers, but you cannot copy a chunk of sentences from another writer without being accused of plagiarism. Paraphrasing doesn't help either. Copy and paste isn't wrong but claiming work as your own is.

Plagiarism can be accidental or intentional. Example of intentional plagiarism is when a post is written in one language is rewritten in another and posted in local board without citing the original poster.
I don’t quite know where the forum stands on accidental plagiarism. When newbies ( I use newbies as a case study because I believe they are more likely to plagiarise although I do not have the stats to prove that) are interested in a particular topic they gather all sorts of information, this may eventually cause problems distinguishing between common knowledge and information that needs citation. Determining what is intuitive and what is external is a battle most face to a point the writing patterns and thoughts feels strange...distant and you realise it’s not you writing anymore!