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Yes, it is standard to recommend that you install an SSL certificate whenever you are hosting public facing filemaker files (as opposed to local network behind a firewall only).
The reason you are seeing that message is that you have the setting in FMS checked off for "use secure connections". FMS is trying to force the connecting user to use a secure connection, but the user does not recognize the server as being secure because of the lack of a valid SSL certificate.
The real benefit of adding one is that it does indeed allow you to secure your server. Most SSL providers have some sort of FAQ about SSL certificates to read, like this one:
Make sure you follow FileMaker's guide to SSL:
And absolutely make sure you purchase your certificate from a compatible provider: