A viewport is how much of the window a view takes up. A view is what should be visible in that viewport.
A good scenario of having many viewports would be in a splitscreen game. In a splitscreen game, each player has their own section of the screen. The game can not render one players view inside another players viewport.
Many viewports can also be useful in singleplayer games, like to render a minimap.
Many viewports can also be useful outside of games, like in a text editor. You could have one viewport for the text, and the other for a toolbar.
Think of each window on your computer as being a viewport. You could be in an application that has scrolling, but that is the view moving, not the viewport. When you resize the window, or move the window, you are moving the viewport on your screen – not the view.
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