SEO is an acronym for "Search Engine Optimization" which is simply the unpaid version of Search Engine Pay per Click, and both fall under Search Engine Marketing or Search Marketing.
Every business strives on customers the same with every website, they strive on traffic and Search engines are one of the mass medium to generate traffic. Getting traffic from search engines only means one thing that your site is being recommended from a highly reputable site more reason you should have your business website listed on search engines like Google, Bing and Yahoo.
Now for an e-commerce to run on SEO only means a lot of reduction with paid search engine expenses, statistics have shown that a lot of web visitors on search engines like Google tend to realize the difference between paid search results and organic searched results, people tend to click more on organic searched results because this websites that have been nominated as top ten results must have passed the search engine algorithms and are deemed worthy to be on the list, unlike paid searched results who typically buy their way to the first page list.
The following points will highlight the basic reasons to the importance of SEO in e-commerce;

1.) It helps the e-commerce site save cost with on paid searched results, even if the traffic doesn't convert at least you get the impression and still don't get billed every time a click is made.

2.) SEO will help you with carving an authority in the niche market via branding of so many related searched keywords in your niche.

3.) SEO will help you to easily target your traffic to the right audience via impression then cut down cost on PPC to target those that do click to perform an action on the site.

These are the 3 most pertinent points to work around with on the merits for SEO on e-commerce, paying for every dollar on a click or impression can really be daunting except you are more of a philanthropist than Google page himself, but look at the bright side a lot is looked upon an e-commerce that is well structured for SEO, it gains a lot of recognition and brilliantly escapes the overhead cost of online advert bills with the rate at which the dollar increase and decrease at the foreign exchange office all those factors won't hurt you if your site is not on forex trading or binary trading.

WHY DO SEO
SEO is a must, it can last longer than your pocket will and if played well you can actually rank for a typical keyword phrase for over a year if competition is minimal.
SEO is a reward placed on a silver platter, if you rank for a keyword phrase you also get to rank for other similar keyword phrases that you didn't even worked on, e.g. “web marketing” and “internet marketing” are the same, you may optimized your site for web marketing based on your keyword research and when you rank top 3 you noticed your also ranking top 10 for internet marketing, that is killing two birds with one stone which equals more exposure and more branding and authority in the online marketing niche.
SEO is a combination of both impression and clicks all for a cent unlike a dollar from paid adverts; although the regular PPC sends you a lot of impressions and clicks it's conversion quality with SEO is not the same, rule of thumb states that top 5 websites enjoy 80% of traffic a keyword phrase generates with the top 3 earning 70% of this traffic, 25% 1st position, 20% 2nd position and 15% 3rd position while 10% goes for paid advertisers (top and side ads, little conversion from ads below page).
This can be more rewarding for e-commerce site that targets local search, because of local SEO, tedious keywords to rank for on Google.com can be attained easily on local search engines and hiring a SEO Specialist or SEO Services Company is a one step forward in saving a lot of penny on every dollar spent for paid search.