Both Wi-Fi and Bluetooth are under the category of wireless technology. SwitchBot Meter uses Bluetooth, but a Wi-Fi option can be easily added.
The SwitchBot Meter uses the Bluetooth Low Energy protocol (BLE) to communicate. So on its own, the Meter is limited to Bluetooth range (e.g. around the house - like a Bluetooth speaker).
If we want to monitor SwitchBot Meters remotely (e.g. away from the house), we need to use them in conjunction with a SwitchBot Hub (Plus), which relays the data of any in-range SwitchBot Meter to the internet using our local Wi-Fi network.
The reason why the Meter uses the Bluetooth Low Energy protocol (BLE) to communicate is that it's more cost-effective and less power-consuming than using Wi-Fi, leading the batteries to last a lot longer (in most cases for a year) and the sample rate of the Meter can be a lot higher than using Wi-Fi. A lot of Meters with on-board Wi-Fi will sample once every 15 minutes instead of once every 2 minutes which is what the SwitchBot Meter is capable of, and their batteries die after a couple of weeks instead of a year or more. Changing battery frequently would be a real annoyance when we're placing the Meter to some hard-to-reach places like a crawlspace under a house.
So with a SwitchBot Hub (Plus/Mini), we can not only add Wi-Fi capability to the SwitchBot Meter but due to the favorable price, we can also have a larger array of energy-efficient Meters pushing to the cloud for remotely monitoring for a lot less cost. As for the Cloud Service, there is no extra fee once we purchase a SwitchBot Hub (Plus/Mini).
We may eventually move in the direction of Wi-Fi once it's possible to have more efficient systems.