-> Overview of the latest changes without having to open additional links. Very handy for git bisects for instance.
-> Automatic generation of change logs.
-> It enforces changes that are purposeful and well scoped.
-> Knowledge stays with the Git repo. No data migration issues if you decide not to use GitHub anymore for instance...
-> And finally... it really just looks nice and professional. I would not take a project seriously if the Git log was just an aggregation of WIPs and Oopsies.