A full backup will create an archive of all the files and configurations on your website. You can only use this to move your account to another server, or to keep a local copy of your files. You cannot restore full backups through your cPanel interface.
1) Something is better than nothing. Ensure you choose remote backup location.
2) Having regular backup of website is important.
3) Incase site gets infected you will have to restore with last taken manual backup. Worst case, your site gets blacklisted, you hosting provider is likely to take your site offline.
You are not going to get time, restoring your site.
And also the fact, that you loose data between last manual backup and latest site content.
4) Even if you do have a cron, the backup is not going to give you details about what files have changed. Having file change summary can be very crucial to website security.
5) Never backup your site and store backup within docroot folder. They are of no help when site gets infected.
Keep backup in off site location.