We can load our entire application in one hit, but is it going to lead to an excellent UX? Probably not. Loading only the parts of your app that the user is actually going to use, then loading other parts of your application later, is a win for everybody.
A given webpage might be quite long, full to the brim with information for the user to scroll through. However, if the user doesn't scroll, all of the loaded data is thrown away once the user navigates to a different page. It would be better to only load the bits that we know the user will view.
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