What's SEO for? What's in it for me?

Those questions seem silly for the experienced marketer but sure aren’t for the medium-small business owner. They are by far the two most asked to me when interviewing potential clients. And, also by far, the two hardest to explain to the uninitiated. I get you. Why writing a bunch of words or tweaking the UI of your website should make you excited? Why would you need to invest in those “words” to get a return (the ROI, return of investment) that is above what you initially spent?