It's common knowledge that archiving data is important, but sometimes we get so wrapped up in our projects that we forget that computers, and hard drives in particular, aren't indestructible. So view this as a friendly reminder to back up those all-important session files, or you could lose the album that you've poured your heart and soul into for the last few months. Plus, archiving older sessions can free up space for future sessions.
Even when taking care to save to a second (or third) hard drive, you can still run short on space. This is especially true if there are many sessions (complete with audio files) on the same drive. A good idea is to archive these sessions. Archiving in this sense means either burning to an optical disk (CD or DVD), or transferring to a backup drive. DVDs have an advantage because they can hold over five times more data: 4.7GB on a DVD versus 800MB on a CD. If you archive to a backup hard drive, make sure to access the drive frequently. Hard drives should be powered on and accessed at least every six months to ensure smooth operation.
As mentioned before, the most important reason to archive is to prevent data loss. An entire drive full of sessions can be lost at any time due to a hard drive crash. Having all of your sessions backed up on other media will allow you to maintain a copy that can be copied back onto a drive. Removable media has the added bonus of being more resistant to data loss (CDs and DVDs don't crash).
So do yourself a favor: Save often, and back up your data as soon as you possibly can. It can save you a ton of heartache in the long run.