What is the P-GW or GGSN and why do more IoT, MVNO and Enterprises have their own?

The Packet Gateway or P-GW, previously GGSN in 3G architecture was something generally in a full MVNE or MVNO or IoT player would have, with resellers mostly using their host’s APN and Data Node. This is changing as data is becoming a key differentiator in many areas, however interestingly becoming less important in others, with many players choosing the simplicity of host Carrier APNs (mobile data settings).