The browser is wired into the OS in real ways, and they matter. Everything from "How does the os manage passwords" or "how are system notifications presented" or "How much screen real estate do I have have on a given window size (scrollbars...)" to subtle differences in how the network stack behaves, or how windows are rendered.
Hell, I can list about 20 differences just off the top of my head - and a damn good part of that is because I've been forced to pay attention to each of those platforms. Which clearly you have not...
I read your comment as "I only use a mac, and I don't understand".