Why Model database in SQL Server is in FULL recovery mode
Why is the model database in SQL Server in FULL recovery mode and other system databases are in SIMPLE recovery mode?


I think you are asking why the model database is different from the other system databases.

New user databases are created with the same recovery model as the model database. The default recovery model for the model database depends on the edition of SQL Server you have installed.

From Choosing the Recovery Model for a Database:


  The simple recovery model is generally appropriate for a test or development database. However, for a production database, the best choice is typically the full recovery model, optionally, supplemented by the bulk-logged recovery model. However, the simple recovery model is sometimes appropriate for a small production database, especially if it is mostly or completely read-only, or for a data warehouse.


Following this logic, editions targeted at test and development environments (SQL Server Express and LocalDB) set the recovery model of the model database to SIMPLE. Other editions set the recovery model of the model database to FULL.