In general, people volunteer their time because it is fun, interesting, challenging, and engaging; because it can help them improve their skills or reputation; and because it is meaningful. Here are some thoughts about how contributing to the Drupal project and community fits with those goals:

Synergy
The more we work together, the more we all benefit from having better software, documentation, translations, and community. Reporting or fixing a problem you have found improves the software, documentation, or translation you are using, which helps you and the rest of the community -- and inspires others to contribute.
Practice and improve your skills
Whether it’s coding, user interface design, graphic design, writing, or organizing, if you’re looking for practice, there’s a task for you! There are also opportunities to practice leadership and management skills, such as resolving conflicts, organizing teams of people, and prioritizing work.
Find mentors and teach others
Working with others on a shared project means you’ll have to explain how you do things, as well as ask other people for help. The acts of learning and teaching can be a fulfilling activity for everyone involved.
Meet people with similar interests
The Drupal community spans the globe, and many people form lifelong friendships through their participation in the Drupal community, whether it’s running into each other at conferences or late night online chats about burritos.
Build public artifacts that help you grow a reputation (and a career)
Drupal is an open-source project, so all of work is public, which means you get free examples to take anywhere as a demonstration of what you can do. Many contributors also look at Drupal project contributions when making hiring decisions for Drupal-related jobs.