Volunteering can teach you a new skill or develop one that you already have. Organizations that are volunteer ran will often ask people to help with something that they are not familiar with if there is a need. Perhaps you are working with a group as a volunteer coordinator because you are organized but there is a need for someone to work hands on with children. You may need to fill that need, and as a result learn how to deal with children and work with them. This type of scenario happens in many groups. You will learn to grow by doing many different activities and tasks.
Volunteering will help you to connect with your community. You will be connecting with those in need in your immediate area. You will learn what your community needs and help to find ways to meet those needs. You may meet new friends in the process.
You will have more options for a career. If you volunteer in your career field, you will make your resume look more well rounded. If you are changing fields and your volunteer work is in the field you are moving to, it can be considered experience. In general, employers like to see that you are a volunteer because it means that you are willing to work, have compassion, and are a team player.
You set a good example for those around you. One person can make a difference, but when that difference is multiplied, it can be life changing. When you show family and friends how fulfilling volunteering can be,you set the example for them to follow.
Material from (16 May 2011): http://www.4laborsoflove.org/benefits-of-being-a-volunteer.html