Getting close to the action is critical in some photography. Nature and sports being two examples.
There are a variety of extension tubes available that can turn a 200mm lens into a 300mm and upwards. But there's a trade off.. And it's a big one.. You'll lose at least a full stop of light and possibly auto focus functions.
Using the same 200mm lens on a Dx sensor, you'll have a 300mm crop (on Nikon) with the lens' maximum aperture.