iCloud automagically backs up all of the data on your iPhone, iPod touch, and iPad. So, if you ever lose, replace, update, or simply restore your device, you can easily get back to right where you left off. Unfortunately, iCloud backup is so automagical that it can quickly fill up the 5GB Apple gives you for free, preventing future backups or forcing you to buy additional -- non free - storage.
Sure, you can go through and delete iCloud emails with hefty attachments, or be frugal about just how many -- and how large --documents you keep in the cloud, but you can also control just exactly what gets backed up, removing anything that's not urgent or important to you, and you can also manually delete old backups from devices you no longer have, or you simply don't need anymore. And that can instantly free up considerable, valuable space for the stuff you do care about.