The platform widget set is a language: when you look at a well-behaving application that uses such language, you immediately know what's a button, what's a menu

The platform widget set is a language: when you look at a well-behaving application that uses such language, you immediately know what's a button, what's a menu, what's a tooltip, what's a label etc., and you can configure how ALL the applications look via applying a theme system-wide. Especially important if one needs certain color schemes or font sizes globally. Modern apps each have their own distinct language, and that puts the user at discomfort (and also danger, from time to time) as he through trial and error seeks how to use the thing to get what he wanted to do done. It's a feature when all the apps look like each other for the most part.