If you're building a IoT project and need to send data back-and-forth, the WiFi module built into newer Pi's should be fine for most cases. But, what if you find WiFi does not have a long range you need? If you're deploying your project somewhere more remotely, without WiFi or a strong cellular network, like a rural area, you'll need another option. Something that can be deployed quickly with no overhead setup and that will transmit/receive over a long distance.