Imagine having an outlet with no signage on it – no name, no glass, nothing. People may come in out of curiosity, looking for product A, when you are actually selling product B. Now, if you have a shop with a legitimate name and presumably a product image on it, it will draw a lot of people from the street, who will ask what you sell and how much it costs. Some of these customers will then tell their friends about their shopping experience, as well as your products and stores. The first example is an example of having a website that is not known by customers and even cannot be found, while the second example is about an optimised website and that’s where the importance of Search Engine Optimisation (SEO) comes into place.
The practise of making a web page easy to find, crawl, and categorize is known as SEO (search engine optimization). It’s all about assisting your clients in locating your company among thousands of others. Any digital marketing strategy should include SEO. It is primarily focused on a comprehensive approach to attracting people to your business using online channels. And to do that, one must ensure the website ranks higher in the SERP (search engine result page).
For any business, advertising is of utmost need. When any business goes online, advertising works best to garner a huge amount of web traffic. SEO gives an opportunity for a great deal of free advertising. A proper SEO makes a website rank on the first page of SERP. And the common belief is people generally scan and review the first two pages of the SERP.
Advertising is critical for every business. When a company goes online, advertising is the most effective way to attract a large amount of traffic. SEO allows for a significant amount of free promotion. A well-optimised website will appear on the top page of a search engine results page (SERP). People, according to popular notions, scan and review the first two pages of a SERP.

Reference:
EDUCBA. SEO in Digital Marketing. [Online]. Available at: SEO in Digital Marketing | Know the Importance of SEO in digital Marketing (educba.com). [Accessed 12 September 2021].