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The protocol extensions documented in this specification apply to situations where legacy FTPS clients are used and/or when legacy firewalls are unable to handle a mixture of unencrypted and encrypted FTP protocol traffic on the default FTP port 21.
Although the Implicit FTPS protocol is considered to be deprecated, there are some benefits it provides. For example, some legacy firewalls might not process a mixture of encrypted and unencrypted traffic over FTP port 21 correctly. Also, many popular FTP clients support Implicit SSL, and a lack of support for Implicit SSL on the server might confuse users and be interpreted as functionality that is missing.