1. You can start an online shop by yourself with minimum time spent on learning things.
2. It is cheap. You have 15 days free then you have to pay subscription which is nearly equal to the payment for hosting only. You may need to pay for the theme around 150 USD if you are not satisfied with free themes. (most likely you will not be satisfied)
3. You can start your online shop within a day.
4. Shopify is limiting you to the way your content is displayed which will help you to enter the content faster. My practice shows that when a user has too many options of how the content can be displayed he/she starts spending too much time to the visual options than to the content itself.
5. Shopify is one of the best first steps towards your online business. Once you understand its limits you can easily create fully functional website on WooCommerce or Magento or any other CMS. Even to the customers who want to create a proper website from the beginning I recommend to use Shopify as a tool where they enter the content for their website. It is limiting them to 15 days of free trial and pushing them to enter the content faster and it is structuring the content to the format needed for the fully functional e-commerce website.
6. Shopify asks you to enter basic things for SEO such as meta descriptions and etc. It is very important that you will enter those things yourself since no SEO specialist can make it better than you. You know your product, you know your customers.
Important thing to say:
Once you realise Shopify limits it will be cheaper to switch to WooCommerce and then apply all the changes you need than to try implementing those changes on Shopify.
Sooner or Later you will have to leave Shopify but none of your work will be lost.