Another great reason to join a student organization is the opportunity to explore your career options. Your major is not set in stone, and the last thing you want to do is graduate, get a job, and realize you hate what you're doing. I once had a speaker in my class tell a story about how he was giving his senior presentation on his engineering project (days before he was expected to graduate) and he realized he could not picture himself doing this for the rest of his life. He ended up going through undergraduate school again to obtain a marketing degree. Joining a student organization could prevent this from happening to you!
Whether it's guest speakers or projects, if you join a major-specific organization you have the opportunity to get information on your currently selected career path. Guest speakers provide you with the opportunity to ask them questions about what they do in their field. Projects give you the opportunity to do the work you can expect to do in an internship or entry-level position, without as much fear of failure or pressure from managers. Some of the students in the organization might already have done an internship as well, and they can share that experience with you.
Whether you're unsure about your career path or you know where you want to go, student organizations provide you with a great opportunity to learn more about your options.