In SQL Server database what are the advantages of using full database backup in SQL Server and when to use it?
The advantage is if something happens you have a backup.
To recover a database you must have a FULL backup. (replication not withstanding)
Depending on several factors including size and how often the database changes, you may wish to take a full back nightly or less frequently. There are many discussions on backup routines.
Main types of backups: full, differential, log
The advantage of a full db backup is that it gives you a complete copy of everything in your db at the time you took the backup. And you can restore that one backup and get the entire db back, exactly as it was.
But, as you can imagine, if you have a large db, making that complete copy can take some time.
Therefore, SQL also allows you to take what's called a "differential" backup. That back ups only the db pages that have changed since the last full back up, which can save a lot of time. The disadvantage is that in order to get back the db, you have to restore first the immediately previous full back up, and then the differential backup.
Based on your original question, I strongly recommend that you hit the books. Backups and restores are the most important thing there is with SQL Server and if you don't know what a FULL backup is for, you're also not going to know about Log file backups, which are equally important.
Seriously, download a copy of "Books Online", install it, and start reading about backups.