While both gtag and GTM’s dataLayer are used for tracking purposes in Google Analytics, they do have certain differences. Particularly, the ecommerce property in dataLayer is needed due primarily to how Google Tag Manager (GTM) is designed. GTM uses the dataLayer object as a virtual layer for storing and managing variables that are used to send specific information to Google Analytics and other systems. The ecommerce property is specially designed for ecommerce data, acting as an attribute for all ecommerce related actions, making it easier to deal with this kind of data. Moreover, providing structured data like the ecommerce attribute in dataLayer can be beneficial in ensuring your tracking code is flexible, maintainable, and scalable.