1. Many clubs and sports will require that you dress up on a certain day of the week.
2. If you have an internship or student teach, dressing up will make you appear more professional.
3. You may have to attend nice dinners.
4. If you are in any type of national or international organization, it is a good idea to dress up when you attend conferences in order to make a good impression.
5. During your junior and senior years, you will be going to interviews where you will have to dress up. Doing so prior to that will help you to feel more comfortable with it.
6. Many teachers (I've noticed this especially in my business classes) expect that you dress professionally for presentations. Even if they don't mention it, doing it anyways will mean you went the extra mile.
7. I know this sounds crazy, but when you dress up, you feel better about yourself and people will take you more seriously.