BigCommerce was used by my previous company as a major platform for building e-commerce websites for smaller boutique online store, with a small to moderate inventory of products. BigCommerce addressed an issue we had with other platforms in that it allowed us a lot more customization options as far as design and themes go.
BigCommerce greatly reduces the headache of setting up an e-commerce site, by streamlining payment, shipping, and SSL certificate options, and having all of those options in one place increases productivity as a developer.
BigCommerce automates a lot of functions and features that would otherwise cause me to spend more time trying to develop and that increases productivity.
Having options in the admin panel for SEO options greatly improves productivity, of only having to install google analytics once in the admin panel and have it reflect on all the pages.
BigCommerce was chosen over other similar products because of the its ability to create custom themes, and tweak the design as the designer and developers see needs. Another plus over other options was that it was a hosted solution so it did not need to be set up on the clients own hosting account. It also gave us file access to quickly uploading and downloading files.
It's ease of use, and the ability to get a site up and running quickly, and its many different themes that are well designed. The admin panels consolidates many options for me as a developer to set up outside of the web development.
I would ask how many products the store will have. One issue I experienced was the issue of scalability, I had a client who quickly became popular and BigCommerce could not keep up with the amount of traffic and business they were doing.