The owner of an eCommerce website will have multiple reasons to use different SEO tools - maybe you want to see how well your site is doing and how well optimised it is? Maybe you're trying to get some links on another site and you want to gauge how valuable they'll be? Either of these things could provide you with very beneficial information. Using just one SEO tool will also provide you with some good, useful information, but using more than one is always going to be a wiser approach.
The thing about SEO tools is that they're not 100% accurate. The only people who truly know how well websites will do in specific search engines are the people who created and manage their search algorithms. Google is the business that most people are concerned about when it comes to search engines and they are renowned for keeping information about how sites are valued private. There are methods you can take to optimise your site to better fit the standards of these search engines, but you can't step back and see how well you're doing as a whole - SEO tools like SEMrush, Moz and Majestic all try to give the most accurate estimation of how well a site is doing, but they are all ultimately a little inaccurate (occasionally largely so.)
It's due to the inaccurate nature of SEO tools that you need to think about using lots of them in conjunction with one another: check the stats on a few and use a little bit of your own common sense to get an idea of what you think the correct assessment of the site. This can then help you to gain a more accurate image of what your site (and the sites which you might wish to be linked from) look like through the eyes of a search engine.