The mouth speaks (and the fingers type) out of what fills a man’s heart. Speech reflects who we are, and what we think, and believe, and what our heart it set upon. The reason for the Bible’s emphasis upon “purity” in speech is simply the reflection of that reality: filthy heart, filthy speech.
Let no corrupt communication proceed out of your mouth, but that which is good to the use of edifying, that it may minister grace unto the hearers. Ephesians 4:29