When the Sync Connector (SC) is installed for the first time on Liferay Portal/DXP 7.2+, SC's tables must be populated using the existing data in the DLFileEntry and DLFolder tables. Otherwise, Liferay Sync clients will only download new/updated files since the installation of SC. This was not necessary on previous versions of Portal/DXP since SC was bundled.
As there is no designated mechanism to only invoke logic when a module is installed for the first time (LPS-101216), SC for 7.2 is using the upgrade process to backfill the data. If auto upgrades are disabled by setting autoUpgrade=false, then the upgrade must be manually invoked.
Furthermore, for modules using Service Builder, SB automatically registers an upgrade step from schema version 0.0.0 to the defined Liferay-Require-SchemaVersion in bnd.bnd. Thus, the only way to invoke an upgrade step for new installs is to register a schema version that goes beyond the Liferay-Require-SchemaVersion.
When LPS-101216 provides a designated mechanism for logic on first install, the existing workaround using an upgrade process should be replaced.