There are many reasons to use the cloud, but the main reasons are convenience and reliability. Saving a file to the cloud ensures that you'll be able to access it with any computer that has an Internet connection, so you won't need to worry about using CD-ROMs or USB flash drives. The cloud also makes it much easier to share a file with coworkers and friends.
With the cloud, you're much less likely to lose your data because it is stored on servers. However, just like anything online, there is always a risk that someone may try to gain access to your personal data, so it's important to choose a strong password and pay attention to any privacy settings for the service you're using.