If you hire a right person then he will target the right keywords for your site
. If your site has related services to searched term by user and your site is well optimized for it then you will see your site at top in search engine result page. And therefore you get many visitors on your site which is actually looking for that service. If you see towards history then you can analyze that traffic made an organization's success. You can also get publicity, revenue and exposures if targeted visitors to the site. Remember one thing that you will get a great return on the investment which you made for SEO of your site.