To be an effective and successful SEO, it's imperative that you're always testing, testing, testing. Why? Well, for starters, the variables that comprise and dictate the SERPs are totally out of our control. How do you know what truly works unless you test? Another reason is that's it's very unlikely you'll ever run into a search engineer from Google at a bar who's so hammered that he reveals Google's ranking algo to you.
But to be more specific, continuous testing is valuable and necessary because: