use of dbms_mview.refresh

I came across this in preparing to upgrade an SE-2 database 12.1.0.2 to 18c, but the question isn't about the upgrade pe se, but in following one of the recommendations of the assessment tool.

The pre-upgrade assessment reported the following:

 6.  Please make sure that all the MVs are refreshed and sys.sumdelta$ becomes      empty before doing upgrade, unless you have strong business reasons not      to do so. You can use dbms_mview.refresh() to refresh the MVs except      those stale ones  to be kept due to business need. If there are any stale      MVs depending on changes in sys.sumdelta$, do not truncate it, because      doing so will cause wrong results after refresh.             There is one or more non-fresh MV in the database or sumdelta$ is not      empty.            Oracle recommends that all materialized views (MV's) are refreshed before      upgrading