I've been asked this question more in the past month than I can remember in a long time, so I decided to do a short video explaining the big-picture only.
For those of you who want to skip to the key concepts, they are:
1) Generally you can easily rank for search terms with low competition
2) Just because you rank first for a particular keyword doesn't necessarially mean that that keyword will drive traffic to your website
3) SEO functions to drive traffic TO your website. It is not the job of SEO to "get new patients" - that's your website's job!