Everything? Really? Let's think about this, what can be ruined by opening your mouth? Relationships. Marriages. Friendships. Car rides. Evening strolls with your honey. A romantic dinner. Even healthy conflict resolution can go south very quickly if one's mouth is moving too much (take my word on this one).
I don't know about you, but once I get warmed up, words continue being formed and escape from my mouth faster than I intend. I often dismiss this "ability" as verbal processing, but God's suggestion is: keep your mouth shut (my paraphrase). I think He'd also agree with my mom, "If you can't say anything nice, don't say anything at all." (if I remember correctly, mom said this more to me than she did my sisters)
Sometimes freedom of speech isn't the best freedom.
I want to experience the freedom that comes when silence is the wiser choice.