If your eCommerce site operates across multiple markets then you will need hreflang tags and you will need to ensure they are set up correctly.
If your site does not have hreflang tags set up or if they are set up incorrectly then it will result in a negative impact on your SEO and user experience. For example, individual store views for different locations may start to compete against one another in the search results (for example a US jeans page vs a UK jeans page).
This can result in lower rankings and/or a poor experience for customers that land on your website. E.g. a UK customer landing on a USA page that shows US dollars and your US stock. This would also impact negatively on your engagement and conversion metrics.