Backing up SQL Server system databases (everything) will restore and recover the SQL Server system in the event of a system failure, such as the loss of a hard disk. The system databases backup include msdb, master, and model. If replication is being used, the distribution database is required.
The backup frequency and retention are usually based on the company's policy. As a general rule of thumb, take a daily full backup, followed by before and after backups when adding a new database or making structural changes to the SQL Server instance.