Puts things in perspective. It does not matter whether you volunteer at a soup kitchen, play bingo at a nursing home for the elderly, help pack food at a shelter, and so on. It quickly makes you realize that no matter how rough things might be, there are others with needs greater than you. Makes it easier to appreciate what you have and get to work on Monday.
Grows and diversifies your network. It's natural for us to connect with people we know; whether it is through work or social circles. Volunteering adds a whole other dimension and diversity because you interact with people with entirely different backgrounds than your own.
Helps sharpen your teamwork skills. At work you're frequently in situations where you have to influence others to accomplish a common goal. How often do you have to do that with resources that don't report to you? Now think about what it means when you are in an environment where the people you work with are all volunteers. How do you get the job done?