Assume that you've successfully completed a Search Engine Optimization and Search Engine Marketing Campaign. Traffic numbers are up across the board and you can lean back and relax...
Not by a long shot. You now have to think about AIDA (and no, not the opera, but one of the fundamental principles of advertising & marketing):
The ultimate goal of any web site is to engage a visitor, and enticing him/her to "convert". Depending on your web site, visitor conversion takes many forms: it might be filling out a form for more information, downloading a trial version of your software, requesting a whitepaper or similar actions. What they all have in common is the revealing of additional information by your visitor, an engagement of sorts.
Yet most web sites experience web site conversion rates of a measly one to two percent. Think about it: 98 percent of your hard-earned site visitors leave without a trace (unless you count cookies ;-> ) Needless to say, even a small improvement in this ratio can have sizeable effects.