Why does academic integrity matter?

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 poor scholarship, falsifying data, contracting work from someone else, or other misconduct means that you have failed to complete the learning process; it is unethical and can have lasting negative consequences for future careers.  Plagiarism, while only one form of academic misconduct, is particularly key because academic work necessarily builds on what has come before; learning to appropriately use and cite others' work is a key skill any scholar should learn.

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