The only reasons would be that the laptop is small and portable. This however has many implictaions. You can access files/programs/games, possibly the internet from almost anywhere. If you want to show other people something you can bring them your laptop, rather then bring them to your computer. A laptop can be used in a comfortable place eg bed, lounge chair ect. It also would free space where your desktop wouldve been, allowing a less clutered workspace.
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laptops are better because you can take one anywhere you want. On the other hand, you also have to keep them charged. With the desktop, you don't.