On a Website like this one with thousands of members and hundreds of daily visitors from all over the world, it's nice to be able to quickly locate and connect with others who share your interests, experiences, and values. Groups enable you to do this.
As a member, you can join an existing group, if it is open to new members, or start one of your own. When you create a group, you can choose whether it will be open to all, or open only by invitation.
So you could create a group for sharing and revising writing and invite a few other people to join you. A teacher could create a group for students to use. Or maybe you are interested in a particular writer or country . You could create an open group to discuss Annie Dillard or the Philippines. You could create one that focuses on cooking or the Boston Red Sox.
These are only a few possible uses for groups.