The setting appears to allow apps to fall back on the iPhone's cellular data connection when having difficulties moving data via the Wi-Fi connection, something that can happen with a weak Wi-Fi signal or a temperamental Internet connection.
Less clear is the purpose of the series of controls below that. There are individual On/Off switches to use cellular data for iCloud Documents, iTunes, FaceTime, Passbook Updates and Reading List. Some believe these are a submenu setting for the Wi-Fi Plus Cellular control, allowing tasks such as downloading music from iTunes or syncing iCloud Documents to require the use of Wi-Fi -- forbidding some apps from connecting via Cellular Data while others are allowed to.
We've reached out to Apple for clarification and as it's still a beta release, restricted to 'developers', the settings could always change to be made more clear or removed entirely.