Shopify is a great platform. No doubt about that. We really love it. However, just like all other platforms it can’t do everything out-of-the-box. If you want additional features or help in doing some of the management tasks then you can either code it yourself – or use an App.
Most of us are not great programmers. And even if we are why reinvent something that is already done? Often it is much cheaper, faster and better to buy an existing solution. That is true for many of the SEO-tasks as well.
Also, there are some things that can only be done with an App. A good example is managing Hreflang-tagging across multiple shops (which, by the way, you get with SEO Master).