Hello Aleksandar,
Full site backups can be used to restore your site even if it's completely trashed. It's a 1:1 copy of everything on your site, files and database data.
System Restore Points cannot be used like that. They hopefully contain the files and data of an extension you are installing, actually the files and data of the extension before you upgrade it. I say "hopefully" because it's not an exact science and most extensions don't fully support SRP backups. Even our more complex extensions don't fully support SRP. SRP was devised as a way to quickly restore components to their previous version if something bad happens right after you upgrade them. They can only be restored fro the back-end of your site. Obviously, if a component upgrade bricks your site they're useless.
I would recommend taking full site backups for serious protection of your site and have SRPs as a convenient, quick method for undoing catastrophic extension updates only.
Nicholas K. Dionysopoulos
Lead Developer and Director
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