Backing up to iCloud doesn't mean that. It doesn't mean (any computer). What it does offer is a means to be able to restore your portable system without needing to be anywhere on the same network as your computer (that typically houses the backup) to do the restore. You'll be restoring from your iCloud that is somewhere on the internet without connecting to any other device you own.
So basically, backing up to iCloud means that your portable device can do a restore from anywhere it has connection to the internet.(At least a WiFi one) Away from Home. At a Starbucks. At a hotel somewhere with internet. Etc.
The only caveat is limited iCloud space, unless you buy more. Thus I believe there are selectable backup options, to only backup certain things to iCloud to save space.
Backing up to your Mac (or PC) is just that. It means in order to restore your backup data, you have to connect to that specific Machine while it is on and running iTunes. Either via USB, or with the new Wi-fi method, if you have that set up.
