The Dropbox desktop app isn't essential for using the cloud storage and syncing service (you can always upload and download files using the web app), but it makes it incredibly convenient.
Once you've downloaded and installed the software, your Dropbox account appears like an ordinary directory in Windows Explorer. Drag files in and out, save files to it, create folders within it – anything you like. Every file you add and change you make will be synced automatically, and will be available from any device when you log in.
Why you need it
When you install the Dropbox app for Windows, it integrates seamlessly with your operating system and other applications. Right-clicking any file on your PC (or connected storage) enables you to send it straight to your account, and lots of common tasks can be automated to save time.
Easy file management
Data transfer management
For example, delve into the app's preferences and you'll find an option to automatically save screenshots to Dropbox, so you don't have to spend time dragging them into the folder manually. Dropbox can also upload new photos automatically when you plug in your camera.
Another useful option is the ability to filter the types of file that appear in the Dropbox directory in Windows. This is particularly handy if you've stored some large files in your account and don't want them to be downloaded to your PC.
If you use the app offline, it will sync automatically next time you're connected to the internet. You can also limit the bandwidth the app uses so it doesn't go over any caps imposed by your ISP.