"You don't need x" is a headline targeting the intended audience, namely people who are about to use x. "Here is when to use x" targets people who don't use x,

"You don't need x" is a headline targeting the intended audience, namely people who are about to use x. "Here is when to use x" targets people who don't use x, but should, typically not the audience. "You probably need y" targets people who might consider y, again not the audience.