Most end users of an application or service experience it through some form of front end. Whether that’s a native application, web front-end, command-line interface, or using an HTTP API, they have an idea of how well it performs. Is it slow to respond? Does it use a lot of device resources? Does it cause the device to overheat and activate the fans to cool it down? In short, if a front end isn’t performing and thus probably has a reasonable environmental impact, a user can feel it.