A previously built website is bound to have errors. Our job is to fix them. Google has numerous algorithms and unless your site is fixed, search engines will not identify it.
If not previously installed this is essential for you as a client to see who is visiting your website, how long they stay on each page and the source of where they came from as-well. All these details make it easier to track the success of SEO by seeing the on-going growth in website traffic.