Search Engine Optimization is important because it allows your site to get a higher rank on Google's Search Engine Results Page for your site's targeted keyword. For example, let's say that your site features movie reviews. Given the nature of your site, you'll need to target a keyword that is relevant to your particular, in this case "movie reviews" or "reviews for movies." Once you have chosen your target keyword, your next step is to optimize your site, so that its contents offer value and relevance to your audience.
If everything goes as planned, and Google detects that your site offers a lot of quality content, then it will make sure that your site gets a high rank whenever someone searches for your targeted keyword. What this means is that whenever someone searches for movie reviews on Google, your site will be at the top of the list of results, and that's a very good thing.
This is because when people look up information on the Search Engines, they usually only click on the first two to three sites. If those two or three options do not have what they are looking for then they will try another keyword, and it won't matter to the searchers if they haven't seen the other results. This is basically why SEO matters: Because if your site is not at the top on Google then it will not get any traffic, and no traffic means no income.