What is desired from digital marketing is that your product or site should be visible to the desired audience and should be able to compete with your competitors in much effective way.
In terms of digital marketing, the availability of your site link should be high in ranking and that is it should be more available on top.
At the same time it is very much essential that to be available more quickly in this competitive environment is one of the important aspect to excel in business.
So, what SEO does it helps to improve your rankings in SERP( Search Engine Result Page).
The necessity of coming in first two or three option of SERP can be easily proved through the fact that many of people just look for first or two pages of SERP and if your site come in these page ranking it would be great to get a decent amount of traffic and then you can also earn through advertising.
So this is the reason why SEO is important for digital marketing.