Theming is a big part of the Linux desktop experience, and icon themes are a great way of customizing it to look how you want. Applications provide their own icons as fallback, but the way their menus, lists, file explorers and other interface elements is in the end dictated by the icon theme selected by the user.
The icons on the notification area are also part of this theming, and it's pretty nice to see icons looking consistent with a theme, particularly when they draw attention to themselves in a homogenous way instead of each application using different approaches.