If executed well, a tagline can do wonders for your film. Even today, you don't think of "Alien" without also thinking of "In space, no one can hear you scream" or "Apollo 13" without "Houston, we have a problem." It's a key part of maintaining the legacy of a memorable movie.
If not executed well, however, a tagline has the power to undermine the entirety of the project. The long hours spent on a script, pre-production, filming, post and marketing can be permanently sullied by a misplaced and uninspired pun in the tagline. And it costs everybody millions of dollars.
Don't believe us? Take a look at the top 50 worst taglines of all time and judge for yourself.