Google tops websites on SERP or Search Engine Result Page that deliver better user experiences compared to their similar websites. when your company is competing in wider area such as a country yo will have more competitors and websites will fight for the SERP. So you need to make bigger moves to increase your success rate in your country.
National SEO is also suitable if you have the resource or will to provide service and product to other cities as well as yours . if you offer services and products country wide you will need to opt in this Service. this is the main difference between National and Local SEO.