As outlined here, users of older iOS devices may have a hard time restoring their devices with compatible versions of apps. iTunes will happily download newer, incompatible versions of apps and trash the older version that would still run on the old device.
Two listeners wrote in saying that restoring their old devices from iCloud will indeed restore old, compatible versions of apps to the device. Actually, it will restore the last version that was installed on that device. So if you never updated on the device to a newer app version because you couldn’t, or didn’t want to, and the developer has not blocked that version from iCloud in iTunes Connect, the device will download that old version again.
The downside to this is that this only works from iPhone 3GS and up. iPhone Classic and 3G don’t do iCloud Backup.