Although it might be tempting to think of plagiarism as only a minor form of cheating, or as a simple matter of academic etiquette, this is far from accurate. Indeed, it is essential that plagiarism should be understood as a breach of academic integrity. Academic reputations can be destroyed because of plagiarism.

Everyone involved in scholarly work, whether senior or junior members of an academic institution, are members of an institutional community which aims to uphold intellectual honesty and transparency. This means that due respect should be given to the originators of ideas, data and works being consulted. An absence of such demonstrable respect through a lack of referencing or other poor scholarship means that you have failed to complete the learning process; it is unethical and can have lasting negative consequences for future careers.  Even if your plagiarism was unintentional, academic work necessarily builds on what has come before, so learning to appropriately use and cite others' work is a key skill any scholar should learn.

So plagiarism matters because you would be letting yourself down, you would spoil your reputation as a scholar, disciplinary procedures may follow, and there might be lasting consequences which could limit your job prospects. It matters too because you would be letting your institution down; it undermines the fundamental tenets of scholarly discourse.