Citing sources is the hallmark of professional and scholarly communication. As a scientist or engineer, you communicate how you built your work and reached your conclusions. By citing sources, you:
Citing sources encourages you to think. By documenting how others' ideas connect to yours, you get the concepts more firmly in your head. Anything else is cheating yourself on your education.
Plagiarism is bad. Whether you content-scrape, buy another's paper, or just don't keep track of what you're doing, it's a breach of professional ethics if intentional, and also a sign of incompetence if unintentional. Either way, be aware of what constitutes plagiarism, and don't do it.