Developers want to push their local branches to GitHub for a variety of reasons:
- so that if their computer dies, their work doesn't disappear
- so that they can work across multiple machines (eg, if they have a separate computer at home vs at work, or if they have a cloud development environment separate from their local workstation)
- to show their work to other people before sending out a Phabricator revision (which they could still do with Phabricator, but some folks feel like they should only use Phabricator once something is ready for formal review)
- because at a previous organization, they always pushed their branches, so they're in the habit of doing it