They make language cooler

They make you laugh 

They make good "advertising techniques" (especially for personal branding)

They make your boss like what you are going to say even more

You can impress any laymen (given they haven't heard them before)

They make you sound more intelligent than you really are.

They may work as euphemisms (let you say the uggly truth in a way that doesn't make you feel that bad about the situation)

Words are misused

They lose their true meaning

People get sick of them

They quickly lose their "groovy" status (except for "cool")

You may look too "wanna be" when you use them

You may sound like you use them too many times to say the same thing, too redundant

If you use them...

You may sound really cool

OR

You may sound lame as f***

But... if you're going to use them, make sure they are still cool.