The widget can be initialized with a few lines of JS, where as the component requires many lines of XML and still uses the widget. So why would anyone use the XML method?
<uiCompnent name="foo"/> in their layout XML.
So, you're correct that the modal widget is easier to get started with. However, if you're building a complex UI Component, and want your complex UI Component to include modal functionality, you'd pull in Modal UI component.
At least -- that's my current understanding of the situation/system. More context in my UI Component series (self link) if you're interested