Every strong piece of marketing speaks to one reader, makes one big promise, and asks for one action. Adding a second message doesn’t add persuasion — it splits it.
Copy that tries to convince everyone of five benefits convinces no one of any. Focus is the difference between "interesting" and "I need this".
Take one live piece of copy — an ad, a landing page, an email — and cut it down to one reader, one idea, one promise, one call to action. Ship the cut version next to the original and compare.