Sometimes these terms are used interchangeably but that is incorrect.

A true backup is snapshot of your database that is stored.  This file is frozen so to speak.  The data in the file is not going to be changed it will only have data until the time it was saved as a backup file, any changes to the data after the backup was made will not show in the backup file.  A new backup file will need to be made for future changes etc.

A sync service is a repository of data that your Informant either uploads to or downloads from depending upon the data in the application.   A sync service is fluid and the data held in the sync account is changed with each sync session that Informant does.

In many cases it is possible to populate or re-populate Informant with synced data.  However since the nature of synced data is fluid, if your Informant malfunctions and broken data gets synced then that broken data will be synced to other devices and if used to repopulate Informant it will repopulate the application with the broken data.

For recovery and peace of mind a backup file should be your go to in the event of a difficulty with Informant that causes data loss or partial data loss.