[pwg]

The point of a VPN when using wifi provided by others is that you do not control the wifi hardware or the wifi environment (i.e., who else it connected to the wifi). Which therefore means you also do not control who else might be listening in on your wifi packets in those environments.

Presuming you have a good password on your home wifi, and are running the better encryption standards, then at home you do control who is connected and what hardware is supplying the service, so you do not have others listening in on your wifi packets (unless you've given those others your wifi passwords).