If a company is interested in getting its website to rank well in the SERPs, then the first SEO Service should be Onsite Optimization.
Some web developers when building your website will add some onsite optimization which can have differing results.
Most will not, leaving it to you, the client, to obtain their own Onsite Optimisation. Unfortunately when you go to an SEO or Digital Marketing company a rebuild is sometimes necessary to fix mistakes which the web developer has built into the structure of the website. If you use a digital marketing company for both you will mostly avoid this problem.
If you would like another company/web developer to build your website and wanted us to perform onsite optimization, then we would prefer to be able to consult with the website developer before the build starts.
By itself, good onsite optimization,( if you are in a low, or low to medium competition bracket) can get you ranked well in Google. Like everything to do with SEO though, you can’t walk away and leave it. Once you have obtained the rankings you want, you will still have to add new and SEO rich content in order to keep it there. Otherwise, someone else will take your ranking.