This allows you to share work without (in Git terms) pushing to master. Branches in Perforce-like systems tend to be more heavyweight and permanent (IIRC you have to branch an entire path of files, it is not the same as the Git concept of "branch" which is just a commit that points to another parent commit).
You can think of the system as enabling you, in Git terms, to create pull requests without the creation of an underlying branch.
Google's system is derived from Perforce, which has the concept of a changelist (think: commit), which can be "pending" and stored on the server for review/cloning by other developers: https://www.perforce.com/perforce/doc.051/manuals/p4guide/07...
This allows you to share work without (in Git terms) pushing to master. Branches in Perforce-like systems tend to be more heavyweight and permanent (IIRC you have to branch an entire path of files, it is not the same as the Git concept of "branch" which is just a commit that points to another parent commit).
You can think of the system as enabling you, in Git terms, to create pull requests without the creation of an underlying branch.