Using sources in writing is a way of developing such a discussion. Writers who plagiarize lose the advantages of belonging to an intellectual community. If they are professionals, they may be barred from practicing their profession or their work may not be taken seriously. If they are students, they will carry the stigma of having plagiarized. Teachers will be suspicious of their work and will be unwilling to support any of their future efforts, write recommendations for them, or even work with them at all.
Plagiarism is one of the worst mistakes anyone can make.
You should not, however, become too fretful about plagiarism. Writers cannot hope to indicate or even be aware of all their borrowings, and there is a point where an idea borrowed from someone else becomes, after long reflection, your own. So long as you are scrupulous about indicating material you have quoted and immediate borrowings you've made in paraphrases, you will not be suspected or guilty of plagiarism.