Funnel optimization starts with copy that sells. It's the copy that has the biggest influence on conversions. Data science findings even show that copy is more than twice as influential as design in why visitors convert.
Copytesting helps you improve sales copy in your whole funnel: landing pages, emails, long form sales pages.
Copytesting gives you data on how your audience perceives your copy, what's confusing or unclear, what questions remain even after reading everything.
Fixing those issues is how you convert more customers.