Knowing which you need.
A DATABASE BACKUP
The content, settings, users and orders. Small, fast, and it is what you need for most problems.
A FULL BACKUP
Files as well: code, themes, plugins, uploaded media. Larger and slower.
WHEN THE DATABASE ALONE IS ENOUGH
Content deleted in error A bad import A setting changed wrongly Data corruption
In all of these, the files are fine and restoring them achieves nothing while risking more.
WHEN YOU NEED BOTH
A failed update that changed code and schema together A compromise A migration A site that is comprehensively broken
MATCHING DATES
For a full rollback, restore both from the same backup date.
Files from one date with a database from another leaves the application inconsistent in ways that are difficult to diagnose.
THE SENSIBLE ARRANGEMENT
Database daily, files weekly. The database changes constantly and is small; files change rarely and are large.
That gives good protection at modest storage cost.
WHAT PEOPLE GET WRONG
Backing up files only, and discovering the database was never included.
