Because they make you sound moronic...
You should avoid slang and informal tone.
It depends on the type of speech. In an informal speech, slang is fine; in a formal speech, you should avoid slang.
Using slang or “filler” words
Slang or filler words
Using slang or “filler” words
Using slang or “filler” words
Chilling.
"Don't sweat it" was a slang expression in the 1950s that meant "not to worry."
Yes, this is true. Messages not delivered in person, should avoid slang unless you know for certain that the recipient uses the same slang as you do.
As an expression "tube" is usually used as slang for television.
The Cockney rhyming slang expression for "wife" is "trouble and strife".
skin, balls,guts