Explanation
The REPLACE option overrides several important safety checks that restore normally performs. The overridden checks are as follows:
Restoring over an existing database with a backup taken of another database.
With the REPLACE option, restore allows you to overwrite an existing database with whatever database is in the backup set, even if the specified database name differs from the database name recorded in the backup set.
This can result in accidentally overwriting a database by a different database.
Reference:
https://docs.microsoft.com/en-us/sql/t-sql/statements/restore-statements-transact-sql