Full form of SEO is Search Engine Optimization. Optimize your website and app to rank high on search engine.
It is a place where people search almost everything and anything online (and yes almost all things are available their).
Any business is complete only when right people (customer) know you, buy from you or take your service, right? So you have to be present where the customers are and search engines (Google and other) is one of the place where number of people search their requirements.
Because your competitor is also on search engine and most of the people prefer to choose and go with the websites rank on first page, very less amount of people go to the second page. 10 results are on 1st page page of Google as well as other search engines.