From an ethical and fair play point of view it’s just not right to use other peoples work without acknowledge it or them. Imagine you play a football game. You score a great goal. At the end of the match someone else claims the credit and they get all the praise. You’d love that.
And there’s actually more to be gained by saying that you consulted other peoples work in your essay or whatever than trying to hide it. Quoting other people is not a sign of weakness or dependency, but a sign that you have researched your topic well, thought about it and read up on it.
Lecturers can get a fair idea of how good a piece of student written work is by flicking to the last section and looking at the Bibliography. They saycan easily tell when someone starts using chucks of text from other people. The style suddenly changes and it sticks out like a sore thumb. It may be an urban myth but I’ve heard it said in some cases the plagiarist didn’t even bother changing or matching the typeface and size.
I do think plagiarism can happen when people are not sure, or haven’t been shown how to reference or cite. "Cite? Reference? Bibliography?" We've a guide for that too. But ignorance cannot and should not be used as an excuse. The “I didn’t know” line isn’t a great case for the defence. Learn now how to search, and research, and tell good information from bad.