Here is what you get when you use a shopping cart provider.
Peace of mind.
Basically all of the backups, bandwidth and network hassles are handled for you. Code only accounts for a portion of an ecommerce site. The other hassles can be overwhelming. Not to mention Security and PCI compliance.
There are other things too; like inventory control, shopping cart abandonment tools, preconfigured access to api's for shipping (UPS/USPS etc), order tracking and CRM to name a few.
The down side always rears its head when you want to do something really custom. Also ecommerce services do a very poor job when selling soft goods like subscriptions, one-off services etc.
My 2 cents. Once your figure your way around, Big Commerce is a pretty nice platform. It gets spendy as you add products. But you can sleep at night knowing that everything is being taken care of.