It's as simple as that. Giving your time up to help other people/causes is pretty selfless. It's a nice thing to do, i think everyone will agree. There are SO many ways you can volunteer e.g. at an animal shelter, for a specific charity, mentoring, helping out at your local charity shop, helping pack bags at the supermarket. The list could go on forever! If you need help deciding on something to do, there's websites like https://do-it.org that can help you find something to do. There's so many little things you can do to make someone smile or to help out someone in need.
You learn.. a lot!
There are so many skills involved in volunteering which are valuable and transferable: Time management, communication, dedication, creative thinking. You will learn so many new skills and reinforce those ones you already have. For example, volunteering at a club for children with autism - i have learnt so much about the condition, how to interact with the children and how to play Minecraft (truthfully, i'm not very good).
You're helping yourself by helping others.
We can't ignore the fact that volunteering is incredibly helpful for a lot of careers and looks great on your CV. So whilst you are doing something that is helpful to others, you're also helping yourself. You're nurturing skills that potential employers or universities will like. Especially as a psychology student i find that volunteering vastly improves my applications, it gives me skills and experiences that are necessary for my desired career path. There are obviously so many other reasons to volunteer, this is just one of them.
It's productive. Stop wasting time!
Volunteering gives you something good and worthwhile to do with your time. Rather than spending 3 hours binge-watching TV series (that is totally acceptable too though btw), try doing something that's going to help your life, help others lives and help improve your skills. Basically all the points above.
Makes you see the world in a different way.
Volunteering really helps change your view of the world. Cheesy, i know. But, you learn to appreciate the little things in life like, making someone smile or helping someone across the road. These little things mean much more to you. It helps to broaden your view of the world by meeting a variety of new people, learning their life views and their life stories. I'm so much more knowledgable after volunteering, i've got to meet a wide range of people and learnt about things i never would have thought of. You learn things about yourself you didn't know already know. There really are things that will broaden your outlook of the world.