Projects are the backbone of organization in Phabricator. They make it easier to find, organize, and manage Maniphest tasks by allowing lightweight structure. Projects can also be used organizationally, to manage groups of people and what they have access to see, create, or edit within various Phabricator applications.
Lists of tasks can get unwieldy pretty fast. Using Projects can better help you keep things organized across teams. Create Projects for things you build and ship, or around security and bug (or features). You can build custom queries in Maniphest, and know just what's going in the projects that matter to you most.
Workboards are built into every Project. Whether you wish to simply track the next software milestone or go full on Kanban Sprint, Workboards help you better organize your team's tasks as you go. Plus, drag and drop is cool. You can also integrate milestones and story points for even more sprint-like fun.
Most Phabricator applications support ACLs (Access Control Lists). This is done more easily with Projects. A simple example might be to restrict people with commit access to your Phabricator hosted repository. Simply build a Project with the appropriate team members and select that Project when editing your repository settings.