Anna,
When installed, Dropbox creates a folder in your My Documents folder called
My Dropbox. Any files you copy into or save into this folder are uploaded/synchronized with your Dropbox account. The files are then available on your Computer and online.
There are a couple of major benefits:
1. You have an online backup of important documents/files that you can recover in case of problems with your computer.
2. You can get to these files from any other computer by going to
dropbox.com and logging into your account.
3. As Fred also noted, you can set up shared files within workgroups or with friends to easily share files.