When you volunteer, you help people by applying your skills and experiences. From another perspective, you actually have chances to practice your existing skills and perhaps learn something new. While there is no guarantee that you will learn skill A or B, you will surely learn something, and it may even be something you never expected!
Working as a volunteer, you will probably improve your communication skills. Perhaps you are an introvert and don’t really like communicating too much with others. By volunteering, you will fortunately have lots of chances to interact with lots of different people during your volunteering experience. You need to communicate with co-workers, other volunteers, students, coordinators in your projects, etc. There is no manual for you to just read-and-follow, so you have to be creative and think outside the box. But you will get used to it!
And of course that is just the basics. You also need to, for example, plan your work. From your transportation, to your work flow, work schedule, and your free time. You don’t necessarily need a detailed list. But at least you should know what you should be doing now and next. If you have no professional experiences such as internship experiences before, or you think you are kind of disorganized, perhaps you will find this helpful for building your professional skills.
Last but not least, don’t misunderstand your purpose - you have come here to volunteer, which means you are here to help and to serve! And personal development is just a bonus.