It may sound unbelievable but that’s what the truth is. You do not need a website to perform SEO. The sole purpose of practicing SEO is to gain visibility and optimization online. It can help you make your presence known in the industry (whatsoever your business industry is) online.
Ranking higher in the Search Engine Result Page for relevant items, searches and queries will get you enhanced conversion rates and ROI. You will have to have certain things to practice SEO but it does not compulsorily include the presence of a website. You can always add that later as per your need and convenience.