Workflows can vary, but what I like:

PR/MR is an "atomic" change (ideally the smallest change that can be landed separately - smallest makes it easier to review, bisect and revert)

Individual commits (or what "versions" are in Phabricator) are used for the evolution of the PR/MR to achieve that change.

But really I have 2 use cases for the commits:

1. the PR/MR is still too big, so I split it into individual commits (I know they will land together)

2. I keep the history of the evolution of the PR/MR in the commits ("changed foo to bar cause its a better approach")