Start with this SEO Definition:
SEO is more than just ranking. It's about educating users, helping them, solve problems and moving them toward a connecting with a site or brand.

The purpose of the SEO: get more targeted visitors to your website which will increase the growth of business. White hat SEO and some natural practices which help the website to get quality ranking results. The majority of the website will place on the 1st page of Google, Yahoo, and Bing.

Coming to the answer! Why should we choose you?
SEO is the technique and trick for increase your brand value in Internet marketing and its one of the best thing for taking impressive place at the search engine.
SEO's working for making the User experience better, SEO's working for taking top position on SERP(Search Engine Ranking Page), SEO work for increase genuine visitors on websites.

Thank you