If you're going to be messing around with custom ROMs at all, it's a good idea to have a nandroid backup at the ready. A recent backup can save you a lot of time and heartache if things end up going sideways. However, backing up your system used to mean rebooting into recovery and waiting for 5-10 minutes while everything progressed. That's not the end of the world, but every barrier to backing up makes people less likely to do it. With Online Nandroid Backup (or Onandroid for short), you can get a full backup done within Android itself.
Onandroid was originally developed by senior XDA-Developers member Ameer Dawood for Sony Ericsson Xperia devices specifically.