It's marketing online. Period.
You're selling someone on SOMETHING. Whether it be opting into a list, clicking an ad, or buying a product and/or e-book.
Unfortunately, especially online, the saying "if you build it they will come" is most definitely not true.
So, to sell people on something you have to have TRAFFIC.
So, that's the game.
1) Find traffic, then optimize it. (zone in your target, etc.).
2) Sell that traffic on your product/service. If not, at least get them interested in your offers, so you may sell to them in the future.
Read step one an two at least 20 times, and get that stuck in your head. That IS what you're after. TRAFFIC.
After that it's simple to understand why you'd want to optimize your website to be displayed in the first spot in google for a keyword that people are searching for in your niche, right?