As one of the more popular features in selectsmartphones, it allows users to make calls via their Wi-Fi connection instead of using the carrier's towers and cellular network. Not only could this lead to potentially stronger signal in homes, but also it doesn't count against allotted monthly minutes.
Why there is no Wi-Fi calling in the new LG smartphone is fairly easy to explain. The Nexus 4 has a stockwith no carrier influence or preloaded applications and features. Simply put, Wi-Fi calling is not built into so it's not included on the handset.
Now this doesn't necessarily mean that crafty modders and hackers won't do their best to get the feature loaded onto the device. In fact, given the target audience of early adopters and enthusiasts, I suspect it will be a matter of sooner and not later.