I've been building web-based frontends for my entire professional career, and my systems architecture skills have been invaluable throughout. Web frontends don't stand alone; they run on server, they make use of networks and networking hardware, they talk to other services on the same server and other server, and they interact with the operating system. Being able to understand all of those moving parts is crucial to producing a high-quality frontend that's not going to fall apart as soon as it gets put under stress, and that will be maintainable and extensible over time.